I've done a lot of the thief guild and dark guild quests, none have been buggy.
As for equipment you can either steal it from shops at night, or buy it (or better yet kill people in their sleep and take thier armor). If you want to get some badass heavy armor early in the game then join the dark guild, after about 5 missions you get one where well... you get a lot of good items lets say (badass sword and axe too).
To me the dark guild has been by far the most fun, thief ones are alright but they dont seem as intuitive as the dark guild ones. Plus you feel like a badass psycho when you do the quests.
In the game I've yet to buy anything, all of my stuff is stolen. The best way to fight people is this, rob them of thier weapons while they sleep, then sneak attack cut them for 6x damage.
The game is really fun in that you can either play it as an honest warrior or something, but at the same time play an assasin thief. I cant stand playing games the honest way so I always chose to be an evil *******, nothing is as fun as backstabbing people who trusted you.