Right Dinx you have a choice of the following engines:
1.6 diesel, 1.6 turbo diesel, 2.0 aircooled, 1.9 watercooled, 2.1 watercooled.
1.6 diesel is horribly underpowered for the weight of the vehicle but all the gear is there for any easy upgrade to a golf gti engine
1.6 turbo diesel is a better choice if you want reliability/economy just about up to the job.
2.0 aircooled was available until 1982 that's the engine mine has, pretty thirsty on fuel (about 20mpg) but very basic and reliable if you get a good one. Mine will cruise at 70mph and I have taken it as far as 85. Most parts are still available.
1.9 watercooled is the most common engine, 8 horse power more than the aircooled engine at 78bhp. They are basically the same engine as aircooled but with a water jacket. They are slightly more economical and you get better heating for the cabin. HOWEVER if somebody has neglected to change the antifreeze often enough or has used the wrong stuff the head studs are prone to corrosion. This leads to leaking head gaskets and is the most common problem on watercooled T25's.
2.1 watercooled is the gti of the T25 group pushing out 112bhp and they are fuel injected. Plenty of power and capable of cruising at over 80mph however they are still prone to the same head stud problems as the 1.9, vans with this engine also sell for more.
In summary aircooled=older van so more likely to be rusty, diesel is reliable but doesn't suit the character of the van, 1.9 watercooled is a good choice as long as it's been well maintained, 2.1 watercooled yes if you can afford one!
Personally I love my aircooled and I've done 8000 trouble-free miles in it so far.
Pricewise anything from about £700 upwards should get you a pretty decent air/watercooled panel van or minibus (mine was £800 with 12 months mot).
If you want a full camper it will probably be about £1500 upwards.
As with anything the cheapest ones are likely to be suffering from the dreaded seam rust.
I would allow yourself about £1000 get a van/minibus the fit a basic interior to your taste.
Blimey sorry for waffling on
