Hey there; welcome back! I myself took a 6 year break from the game, so hopefully I can offer a useful perspective...
1. High seas looks interesting - is it and where can I read up on it? Would I need to upgrade to S.A. and would I need to use the 3D client? (I'm using M.L. & I hated 3D and quickly went back to 2D originally)
The game still runs with dual clients, but what was the 2d client is known as "Classic Client" to the players now, and the "Enhanced" is what has replaced "Third Dawn" (and a client before that, but that's another story!). Neither limits the places you can go nor the things you can do in game.
If you've played anything else in those years away, I would recommend you give the Enhanced a try (in combination with the Pinco UI, we can set you up with that later, but it's a sticky in the EC forums if you want it) as going back to
literally how the game was may not be enough... the EC has a considerably greater amount of usability than both old Third Dawn and the classic, and is being updated with constant player tweaks. It's also due a graphics upgrade "soon". "In two weeks" etc.
However plenty of players still swear by the CC, and if you want that true UO feel, don't hesitate to use what you prefer. I would recommend having both installed, as the EC is free from UOHerald, and every now and then bugs for one client or the other do still pop up. It's good to be able to have that log in security!
As for High Seas... it's a matter of considerable debate. It will give you a 20% housing storage boost, access to the new ships and content, fishing quests, and a little extra in cooking. However, it does not open up Stygian Abyss content, you will need to upgrade to that separately. SA does include all previous expansions though.
Ultima Online: High Seas - UOGuide, the Ultima Online encyclopedia
If I had to choose between the two, I would say your best bet is to go for SA rather than HS. The reason being that SA opens up almost everything, in particular Imbuing which has replaced pretty much everything else with regards to how you equip your character. If you don't have time to develop your own character, it still let's you go to all the other content (Underworld etc) since you played, and equip every Artefact etc on the market bar a few from High Seas which you can completely live without.
Would I recommend High Seas on it's own though? Tricky one. Everyone gets the new smooth boat movement, so you don't need HS for that; but the content is still being balanced. Fishing quests as they currently stand for instance are an incredible boring grind to get to the 120 scroll, few people have them these days hence they go for multi-millions, but this is being looked at again soon. But it's something you can enjoy by yourself, and mostly quietly (sea serpents remain a pain up the nauticals after all this time though!). If you like the idea of catching and mounting trophy fish, this may be an expansion you want.
The ships themselves are fun, and fighting pirates can be exciting; but even with the changes to how the Saltpetre for ammunition is gained it's still time consuming to make your own ammunition, few people are selling it, and ship repairs after sailing 20 minutes each way for fighting pirates is ungodly expensive, so hardly anyone bothers now; If you've got a group of friends/guildmates who want to do it, or you already have a GM or above Alchemist, Tailor, Smith and Carpenter (you need all 4) you can do this content solo on your tamer or a competent mage/archer certainly. But on limited time it's probably not going to be for you.
On the other hand, the new Event content seems to be pretty popular; Corgul and Osireadon are constantly summoned, and although neither are soloable/husband & wifeable there seems to be a good community spirit in announcing them so an individual can wander along to them when they've got the time.
See how you feel about the game first before investing in High Seas would be my advice then. You don't need it, but it can be nice to have if you can work around it's design problems.
2. Would 10 hours a week be enough to have good game play? (At the end of my UO life my time used to be taken up running quite a big shop, crafting, taking my tamer out and fishing/TH - hubby was the one that did doom etc)
Short answer; Yes. More so than it used to be in fact, as the lower playerbase has opened up space in the economy for even a small trader. Ask around for a vendor spot in Luna... usually you can get one for free on Europa.
The same goes for game content. All the old content is there, and will be just as challenging if you don't know how to gear for it yet! And when you do, much
much easier. If you got to 120 on your tamer for instance you can eat the old dungeons alive now. And people are happily soloing Doom these days. There remains the Underworld and the Abyss if you want tougher challenges though, and the new Peerless system are group events.
The trick will be keeping it to
just ten hours a week, as there's now nearly 15 years of added content to the game. And 15 years of gold inflation; if you want to gain some of the really rare items, even for sentimental reasons, be prepared to invest at least some smart work into catching up quickly. One of which is...
3. Before I learn the hard way - ie dying and losing a load of stuff! - are there any other big changes I should know about?
You mentioned Doom, so you must have been here for Age Of Shadows. That means you know about item qualities and Insurance? Those remain the same, but as I said Imbuing has basically ruined the concept of checking monsters for loot; now you just unravel all the loot into magical powders and then make your own items with the properties you want.
Imbuing - UOGuide, the Ultima Online encyclopedia
In theory monsters now can spawn more modifications than you can make, but as the Weighting remains, and the RNG will always roll 6 mods you don't actually want together, people do not bother.
Find yourself an imbuer in game, or make yourself one and that is the main change you will need to be aware of.
Beyond that, your tamer wants to investigate the new, high end pets that are available. The best for solo play is the Greater Dragon, which is now stronger than the Ancient Wyrm (who remains sadly untameable!), although in some settings a Cu Sidhe will be recommended, and I am one of the few who really seems to appreciate the Bane Dragon; it's nearly as good as the 4-5 follower slot mounts, and combine it with a Nightmare for the best damage output, if much weaker tanking than a Greater Dragon. (Mare/Rune Beetle is not far behind)
You will hear pets referred too as "x Star rated"; they are talking about this webpage:
Pet Power Calculator | UOCraft.com
Anything over 4 stars is desirable to the purchasing community, and will give a noticeable, but not overpowering bonus to it's performance. Anything over 2.5 stars is good though, just don't tame yourself a muffin!
You can have 7 character slots now... as someone on limited time I would recommend getting this separate upgrade, as you will not have enough skill points on 5 to cover all you need, if you wish to keep any old mules you have too.
Check these forums often, as EM events are a regular occurrence now; it is dynamic content and, if you are on limited time, the best way to catch up in gold with the veterans. Every now and then the event will give a unique reward, and with only about 30 of them on average being collected you can sell for 5 million gold upwards, depending on desirability. Watch Europa Trader in chat after every event and add a few million a few days after he stops asking for the items for their worth to people
EM Events are also fun ways to see more of the game, and places you could never normally see otherwise; they are also slowly adding unique history to the server again (EM Emile recently put a monument to a guild member who died last year outside where the Sosaria Red Cross first formed) ... it's not always to everyone's taste, and the events sometimes don't work out, but they are rather unique at least.
The main obvious change which you will have seen, is the inclusion of World based chat. Use this to keep in touch with others in game; however you also need to click User Options and go to the Ignore tab to remove the constant PvP epeen waving I'm afraid to make it bearable! Select "Add From Chat" and then the name. Be aware this prevents you seeing them spell casting in Felucca too... Once done though feel free to ask questions in General Chat!
And if I am in game I will try and help further too