You wouldn't be replacing roof you would resealing failed joints & seams like awning rail or roof light perhaps. On an older caravan most owners will diy because at commercial rates £50/hr or more+vat its not really worth doing as these sort of repairs are time consuming.
If you are considering an older caravan you need to check it with damp meter before you buy. Its unlikely you will find one perfectly dry but sealing up outside with PU sealant will usually do the job to keep an old caravan going.
think ill spend a bit more - I like the folding vans could have picked one up at 700 notes but its had roof repair done so it worried me the other one looks near on perfect on at 1900 delivered a 1991 model...:D soo excited about getting my first van, we looked at a comfy gobur but 1986 model but the lady wanted 1500 I feel that's a bit high for a 30 yr old van...
If you are after a folding 'van like Gobur or Esterel then you have condition of gas struts to consider as well as they are not cheap to replace. If gas struts become weak then it is difficult to raise roof & so on.