The problem with granting class and level to NPCs is that it will not be edition neutral, even for D&D. My suggestion is to give NPCs a general description as to capabilities, even going insofar as ranking them if the character has a real reputation. For example, a guard may be described simply as mildly competent with a spear and shield, but with little actual combat experience. Or a famous warrior might described as one of the best swordsmen in the land, likely able to best X and Y, but probably not Z in a straight sword fight.
Then, if you really want to do the work, you might note race, level and class for a few editions of D&D (probably Pathfinder and 4e), and perhaps describe the more noteworthy equipment and spells they would normally carry. I wouldn't waste time noting ability scores, hit points, or those kinds of things for each NPC. Maybe include a few tables in the appendix covering combat stats for each class & level used of the editions you want to cover, just so DMs aren't caught with their pants down if the players start a fight.