Indeed, resetting levels won't spawn more upgrade scrolls or Strength Potions. It just "dries up" the "reserve" for the zone you are visiting, which somehow seems to be predetermined. I still managed to get almost every single ring and wand this way - if you managed to beat Goo without wasting upgrades on items, you can keep repeating it to get a +1/+2 item of your liking which you will then proceed to upgrade.
Anyway, I finally beat the game with a Mage as well, completing all three classes. My problem with it is, I played it like I would play any other class, and there is no significant difference between the two classes - the magic missile wand really helps only in the first few levels as damage scales horribly and there's no point wasting upgrade scrolls on it.
The only wand that really makes a difference is the Slowness one (and perhaps Blink), but nothing beats Plate Armor and a properly upgraded tier 4-5 weapon. Period.
Also, Paralytic Gas potion + Poison Cloud potion = dead boss, always. I killed the Dwarf King this way for giggles, and I had fun inflicting every possible negative status on him while watching his HP draining from afar