Suggestion and rationale:
Alchemy - able to mix restoration potions. More levels helps ensure you won't run out of potions midway.
Elemental Arrow - Does random lighting/fire/cold damage with one shot. Useless at the end of game though.
Repair - At least one level - having your bow or sword breaking at the wrong time is a major disaster.
Enchant - No doubt about it -- have it at level 5. The game may get too easy after you do so though, and getting the capital for gold charms may prolong the game.
Lockpick - What decent thief does without this skill? The wonders beyond locked doors are yours. Hint - I remember that you can get 3 levels of lock-pick for free in the game.
Deadly Gift - Thieves don't go wading into tides of enemies. They lure enemies into traps of death! Scorpions can be quite expenisve and rare at the beginning, so use sparingly.
