You seem to have already made your mind up but to bring a 140k mi x5 back to like new (my definition may be different than yours) the juice doesnt seem worth the squeeze unless your turning all the wrenches yourself and can find those missing parts free or inexpensively.

I guess I would start by defining "all" maintenance records he has and proceed from there if thats just oil changes and air filter swaps...then you need to make a decision.

Jus my .02. Good luck