How should I set up a draft from multiple old sets?
I'll extend my comment to an answer.

I would do the following: Sort the cards according to their color, plus one pile for multicolored, artifacts, and nonbasic lands.
Then make sure that each pile (except for the land pile) has around 70% creatures. 
If the color piles are of very different size, then I would randomly remove some cards (or you could fill up the other piles with cards from your collection).
From the non-creature spells, try to keep removal and avoid silly situational cards like Mudhole. Then make booster packs of 15 cards that roughly contain an equal amount of cards of each color. With those booster packs you can then continue to create the limited format of your preference.
Personally, I would ignore rarity completely.