UOSteam isn't a legal option to play with on official servers. If you want to play you have to downoload the official client here:
uo.com
UOAssist is the only approved third party helper type program at this time, which you can get here:
UOAssist
UOAssist is not required to play. But it is helpful if you play 2d. I would play prior to purchase to see if you even want to get it. The macros for 2d have been updated recently so UOAssist is (in my opinion) only good for mouse macros and adjustments to displaying info at the top of your screen.