Consider Old School Essentials: Advanced Fantasy. Has all the races and classes from AD&D, except the monk. Has the option of ascending armor class and base attack bonus. They are on the stats for every monster. Easy to modify if you want to add shit like feats. Based on Moldvay/Cook B/X D&D so easy rules.
Spells are simple affairs, mostly one paragraph descriptions…mostly. But you can import spells from AD&D and give them to the players as treasure, scrolls or a captured spellbook. Or not, if you want to keep things simple.
All the monsters from old D&D are compatible, so if you have old monster manuals, fiend folio, creature crucible, or any of the OSR monster books you can plug and play.
All the numbers like Attack bonuses, Armor classes, plusses on magic weapons, and player character hit points, are kept to a minimum (Fighters, barbarians, paladins and rangers get d8, clerics and druids, d6, theives, assassins, magic users, and illusionists:d4) and they stop rolling at level 9, and just get a flat number (+1, or +2) each level after that.
Only 14 class levels, so make each level count!
Hardcover books are $40. For Player’s Tome, $40 Referee’s Tome and when you buy the book, the .pdf’s come with it. If you buy .pdr’s only they are $15 per book.
Frankly if you have old D&D books you only need the Player’s Tome for classes and spells. I only bought the Ref’s Tome to support the product, and because I am a hopeless collector of game books.