1. Potions are completely useless, they are not nearly as abundant as in the other TES titles. You can rest to heal or purchase Heal spells.
2. Don't bother with trying to get a full set of armor. Just wear what you come across (and of course what your class allows you), even if you end up looking weird.
3. As for the dungeons - you do have a Recall spell right? If you get lost just teleport back and start again. Also the dungeon random generator probably didn't work as well as intended as there are sections of dungeons that are completely cut off from the rest and sometimes the quest goal is in these areas. If it's a side quest, no problem, you can get back the lost reputation easily enough, but if it's a main quest... My advice is to save before picking up quests and if you can't find the goal then just load the save and take the quest again to get another dungeon.
4. A bit lame, but you can create an extremely weak open spell in the Mages' Guild and it will open any shop door without the guards noticing. Then you can just rob the shop and sell the wares you stole from them right back when they open up.