Which.....
begs the question Christian, is this something you want to use or just collect? Are you wanting something with the most collectable status or something you really fancy like the G?
Antique vacs like the XXX only have value to other collectors. Some owners have just given them away. There's not a good set of reliable prices on old vacs....it's whatever you can get for them. So buying a XXX isn't an 'investment' for sure.
On the other hand if you find one in an original case/box with all the attachments and in pristine condition....obviously on eBay, someone will pay well for it.
I say figure out what model you want and then start your search. A long time ago when I first came here, I mentioned a desire for a 1205 and a member sold me one for a very reasonable price.
You never know. I have many un-restored G's. I'm not going to restore one and sell it, because that would imply there is some sort of warranty whether it's stated or not and hard feelings could ensue. I would do that locally, but not LD. I used to restore Diamond J's and give them to friends and families. There's hobby fun and then there's business and if you're really lucky, there's both.
Kevin