I agree with John L in that a project on a van as old as this could be a one way street to financial ruin. Yes, people do restore many old cars, caravans etc but they usually have technical skills and hopefully access to equipment that will help them in some of the parts.I am not stating you don't have these, merely that it is not mentioned and by implication I feel you are an enthusiastic amateur.
Apart from the chassis comments made by John I would also add getting a professional damp check done on the van. If this shows damp then you have to seriously consider what you do next. It may be repairable, or it may require a complete rebuild. If this has been done and was fine, then ignore that comment.
Replacing curtains, carpets and even upholstery is probably the easy bit. What about the equipment? If it is original it is getting on for over 30 years old and will be well worn. If there is a fridge check it carefully and probably a replacement will be required. Similarly with the hob. Any electrical wiring could be suspect as well. If an owner has done 'modifications' over the years, these will all need to be checked out. I cannot remember but if the van is that old you may need tocheck if the towhitch is 50mm or 2 inch. They look the same but are definitely not. Check also the window rubbers. The windows wil probably be glass but check for cracks.
You will also need to check the tyres carefully as again if they are old the sidewalls willbe cracked and suspect.
Bluntly, as John has commented even if the van was a free gift I would still be suspicious, unless it has been lovingly cared for. It has value of........zilch and you could easily spend several thousand pounds and once started, it is difficult to stop.
If you are still dead keen then find the owners club for the make of van, join that and get some ideas.