I'd be wary of 40+ year old amps. Have they been serviced? Any sort of guarantee of function by the seller? (You didn't mention if the seller is a commercial outfit or an individual--the electronics repair shop in my neck of the woods warrants the used kit they sell--but if you're buying "as-is" from some rando, you should pass.)
The amps themselves were innovative enough for their day. They were affordable, conservatively rated, and performed well. Under the hood, they sported a class G supply, which they exploited quite tirelessly for marketing purposes, touting their rather prodigious peak power via IHF headroom specs. The 3150 was rated 50 watts continuous, but with >6db dynamic headroom, or 200+ watts or so. They really could do that, too, and for far longer than just a few cycles. And they could drive low impedance loads.
That was the good. Now for the reality. At their age, they almost certainly need re-capping of the supply, definitely need re-biasing, and probably a whole host of other repairs and maintenance. Dated parts in need of replacement may be unobtanium. While serviceable, they are a real rat's nest in there, too. And the cost for all that makes the whole idea a poor value proposition in this cheapskates opinion.
If you must go vintage, your son would be better off with something a bit less vintage than those ancient NADs. Maybe a early 2000's Yamaha instead? Or just get new kit, depending on how he plans to use it (e.g. streaming as a source, or bt, etc). The JBLs are probably sensitive enough that Yamaha's least expensive two channel networking integrated would suffice. Just make sure it's one with that retro-cool (and genuinely useful) variable loudness control!
