Sure they'd be a good buy, but as DC pointed out, gear that old will very likely require a tune up. My parents have a Marantz 2275. Still works, but more than half of the switches are shorting out so they rarely use it anymore. Also, my understanding is that for some of the parts on those vintage Marantzes, you have to canibalize other units for parts. A good audio repair shop should have a lot of stripped out parts lying around, but those shops are few and far between.

Also consider that the 2275 cost $600 back in 1976. In today's dollars, that equals over $2,000. If you compare what you can buy for $2,000 today versus that Marantz receiver, it's not even close.