Personally, I think getting more health is a top priority in almost any roguelike. Note that potions of increase hp/mp become less effective the more you use them, so they're not as good as they used to be.
Deemer's advice is good though. An amulet of reversal is a great tool for massively increasing your versatility. The amulet of undeath is good as well, but sometimes you can't find as many amulets as you want or you just have something else in mind for them. I mix my extra potions of death with potions of inversion or dissolution to make more potions of life for that. Energy charged crystal balls are fantastic and everyone should use them.
Life magic is a good spell school, but I think you could get through the game with any combination. Armor lets you get away with having fewer hit points, and heal is really nice to have on any character, but I think that building your character around your school choices and playing them appropriately is more important than which schools you choose. Just as an example, the elemental school gives what I consider to be the best offensive options in the game, so taking edges like powerful bolt is redundant, and you would be better served by using those on spell training, or something to improve your defense.
Cosmic is a great choice for any character because once inverted, you can haste yourself to double speed for only 1 mp.