Under $40 for a complete "decent" cordless drill? Sorry but I don't think that is going to happen unless you get really lucky.
I splurged and bought a dewalt XRP or some letter combo

~14 years ago. My first power tool

Was something like $150 back then, alot of money for a 11 year old. I still use it all the time, however the batteries are showing a significant shorter useful time between charge. The drill itself works fine though. Only a measly 14v compared to today's monsters, but will still put a 3/8" hole through steel without sweat.
I would recommend that you get yourself a nice drill, new, unless you find one SUPER cheap. Something about buying a used battery just doesn't sit well with me. Most of those batteries end up costing more to replace than the drill itself.
Then buy your dad a corded, used miluwakee or the like, something that he can beat the snot out of and not worry about breaking.