It took a good while with me, and at 5 gold a pop, the costs build up. Yeah, it's doable, and a dedicated player will get there in not too long, but the thing is, I think the mage class should be unlocked pretty quickly. It's the funnest class so far, its playstyle is unlike the default classes, and it's a popular role to play in roguelikes and RPGs in general.
Basically I think people would have better impressions of the game if they could try the mage class out, and therefore it should be made available very early on.
Edit:
Two bugs:
First, the boss of Amon Sul isn't spawning for my current character, although he has in the past.
Second, the level five sense spell is not displaying the locations of traps. It lights up the squares where they are located for one term, but this is not particularly useful, especially since it has no effect on an already-lit tile.
Edit 2
If any of you kill a boss, and it drops its heart as an item, you want to eat it. Trust me on this. The game will talk about it as though it's a questionable idea in the item description, but it's really good. You might decide that the 400 gold you get for selling it is better, but it's worth doing it at least for the first time.