I'll give you my opinion on some of your questions...hope it helps
archery or fencing: both are legit for dexxers
to help you make a decision I would take a look at your ping. If you ping rather low (50 or less) to Siege you can absolutely use fencing (or any melee skill). If your ping is more around 100 ms then I would recommend archery. I play from Europe, so my ping is usually around 150 or worse. I use archery because that way I don't have to be able to be right next to my opponent in order to hit him. People still outrun me but at least I can get some shots in before they're off screen.
also using the EC will help with being a dexxer - it is faster and offers a lot more and better options for macros for dexxers.
1. the build
again this is somewhat determined by your ability to outrun opponents when in a pinch.
hiding, stealth and ninjitsu is an extremely strong defensive combination that will allow for great survivability (when low in health smoke bomb, dismount and stealth away) and this will work even when you have a high ping in most cases.
If you feel confident that you can run fast enough then samurai builds might be something to look into - evasion gives you great defensive abilities, especially when combined with parry, confidence heals on the run make it hard to kill you, especially when you use the bushido mastery that makes it impossible to interrupt them. I would recommend swords over fencing for this though (bokuto!)
there are numerous options to combine skills, so this can't really be answered that easily. poisoning is a strong choice for fencers (and swordsmen), alchemy can help with healing and boost damage from conflag potions etc.
both mysticism and spellweaving can be great
chivalry while a great skill kinda is a rich people's skill though because it only really works with 4/6 casting in pvp which is costly to put on a suit in addition to the usual hci,dci, etc. etc.
there are so many things to consider when choosing a template that I would really recommend talking to someone in person and trying things out and adjusting over time.
2. the top 3 macros
you will need a lot more than 3. as a dexxer you absolutely want to have a target closest mobile, attack current target macro. then you need macros for refresh, heal and cure potions, plus all other types of potions you want to use. If using skills that have spells or abilities that need to be activated you need to key those. E.g. for ninjitsu you need to key smoke bombs, animal form, death strike, focus attack and shadow jump at the very least. you won't have time in combat to double click icons and you need to be able to use them on the run. So as you can see this is largely determined by your choice of template.
3. equipment
each template has certain minimum requirements for equipment. as a dexxer you absolutely must have 45 hci and 45 dci on your suit and weapon combined. also you need enough stamina to swing your weapon at max speed, enough mana to be able to use lots of special moves and 70 in all resists.
On Siege you only get to bless one item and the item based combat system forces you to farm items and ressources to support your pvp habit. Pick your Siege bless item carefully and build the suit and weapon around it.
a typical dexxer suit will have 7 stamina, 7 lmc, 7 mana (among other mods) on each imbued piece of armor to get you to where you can swing your weapon as max speed and use a little mana as possible for special moves. since you will loose your suits you want to keep them cheap - hence 7 stamina, lmc, mana because 8 would require special ingredients. again the suit is determined by the template, so it's kinda hard to tell you the exact suit you will need without knowing the template.
I absolutely recommend having 120 imbuing on a soulstone and a character (or skills on soulstones) that allow you to farm basic imbuing ressources and ingredients. PvP templates are usually not too great for farming, especially the dexxer templates.
4. other tips
get with a group of pvpers - it is so much easier to have someone show you the ropes than having to find out everything for yourself
make sure you have the ability to do highend PvM with either your PvP char or an alt. PvP on Siege is expensive!
on Sundays there is usually a VvV PvP event that promotes a no looting agreement. this is supposed to let newcomers gain experience and try out their builds without loosing everything every time they die. Over the past months attendance has varied with sometimes not enough people to have fair fights. It starts usually around 2-3 pm EST and usually the 2 biggest factions are GIL on the one hand and the Vice alliance on the other. Occasionally there will be people (LB, Hi5 and the likes) who don't respect the no loot agreement though, so be careful.
