I think you are on the right track, but imo you won't lose the body part when purchasing it or restoring it if you can do the work yourself.
Try to keep it in any condition able to be used and enjoyed even if it's a bit 'rough', try to not lose the ability to actually camp in it even if it's a 'work in progress', b/c everyone that owns one is almost always in some state of completeness, aka incompleteness.
If you take one on, tear it apart, and then lose the ability to take it out camping you could become discouraged and then end up quitting on it. It's sad when that happens and it happens a lot imo.
But in general, yes. Compared to a new Class A motorhome you will get way more camping trips, miles, smiles, memories, .... (everything that matters) per dollar from a vintage GMC motorhome than from a new motorhome if you're willing to live with and enjoy the GMC the way it was intended in the 1970s. Things in new ones are not the same today, and I don't mean they are better. The designers made the GMC a great motorhome, way better than camping in a tent. But they aren't for everyone, some people want 120 gallon water tanks. Leave those people at home.
If you get a GMC understand that they won't be troublefree, things can and will break. Have a contingency plan such as some emergency cash just in case you're on the road and something happens, but that's not unique to the GMC. New ones often leave the dealership broken!
The new ones break down on the road too, maybe even more frequently, and when they do forget about any warrantee. The dealers only care about getting you separated from your money on day one, not about any follow-up care. Best case, they fix your new motorhome but it takes months. Meanwhile you are paying the monthly payment and not able to camp or travel.
Be aware that you might not resell it. They tend to grow on you . In fact, you may end up buying more than one by the time you're all done.