These are old classic vehicles and will need regular maintenance to improve reliability. I suggest you spend time learning about the van especially in the engine bay and underneath. Knowledge will be important. Get a couple of decent manuals. The Fletcher Gillett one is particularly good. As said replace all fuel lines with what's recommended on here and change the engine oil and filter. Check out the brakes to make sure they are in good order. Check out the tyres while you are at it. Look beyond the nice paint and interior.
mine are over 10years old on the van and in perfect condition and I will not be changing. Unlike the 3 year old ones on my car that are showing signs of cracking
Although they may look ok on the outside the rubber hardens over time. Ours had some 12 year old Michelin Agilis on it. They looked nearly new with loads of tread. I Changed them for some new Hankooks and it totally transformed the ride quality. A much quieter & smoother ride. Manufacturers recommend a life of 10 years on tyres, but it’s your choice.
Plenty of info if you google it. Although not law on cars yet it’s now illegal on busses coaches etc. As has been mentioned before the new tyres you buy could have been in storage for several years before you have them fitted. Difficult call but I think for safety’s sake particularly on a 40+ year old camper with no airbags etc I would rather have decent Tyres than take the risk. https://www.petebarden.co.uk/news/articles/how-old-are-my-tyres.html#
but who says new are more decent, like anything, its been mentioned due to an accident with a very very old tyre on an actual bus, im quite happy to go off condition the same with my brakes suspension etc, if its new and broken it gets fixed if its old and not it doesnt