Firstly Hi and welcome to this place from us in sunny (at last) Chester
You will find LOTS of info on here
I would say do you prefer Diesel or Petrol ?
The 1600 Diesel powered ones are very slow and the engine will have been thrashed by previous owners trying to keep up. The 1600 Turbo ones are better but still suffer as above plus turbos can be a cause of probs and are not cheap. Many folks fit the 1900 engines to their vans and apparently this is a very worthwhile thing to do. (though I am not suggesting you buy a van and
then do this, the cost would be large - it's normally done after the 1600 engine has worn out.) They all are much more economical than an engine running on Petrol.
Diesels stink ! (imho)
Petrol engines.
1600 Aircooled = under powered and weak.
2 Ltr Aircooled = OK on power, expensive to fix if it goes wrong.
The heater system on Aircooled hardly ever seems to work properly.
1900 watercooled = OK on power, all round these would be my choice (and is

)
Expect to cruise at 65 to 70 and average 20 to 25 mpg.
I thoroughly recommend having an LPG conversion fitted, many owners of these vans do this.
It results in roughly half price motoring (ie same as 45 to the gallon) and has no down side I can think of.
Costs around £900 and pays for itself in no time.
They can and do have their own "special" problems and when buying I would take a "WBX" ( = flat 4 water cooled ) expert with you to check the Head to liner seals, the condition of the water, oil, plugs, compressions, exhaust (£400+) oil pressure etc. Reason is if these engines fail you are often better off just going for a £1000 recon unit from Elite rather than stripping them and repairing them. Even a blown head gasket often than not results in a new engine for reasons I won't go on about here (yet)
Get a good engine and it will go for a long time and be very nice to own.
A lot of the above may be influenced by what usage you are planning.
Like daily driver versa 6 times a year off camping, rest of the time sat around (the worst thing to do by the way).
It has been said above, when buying a T25 the body is as important or perhaps more so than the engine.
Look for rust in the outside panel seams = bad news
Rust around any / all of the windows = bad news but not a fortune to fix.
Rust around the filler cap = bad news but not a fortune to fix.
Rusty rear wheel arches = bad news but not a fortune to fix.
Rust in the outside edges of the voids (spaces) in each side of the engine bay behind the rear wheels by the outer panel. (they often go there and many owners are not aware of it. If you find rust there the owner did not know about, it's a good way of haggling the price down

) - not a big problem, just usual body work stuff.
Keep the Q's coming.
Before I bought mine (5 years ago) a mate told me driving a T25 is a good stress buster.
He was right. You relax in them. You know you can stop almost anywhere for the nights sleep or just for a snack and a brew. The holiday starts when you set off, not when you get there
Oh one last bit of advice (for now), don't buy the first van you see.
There are plenty out there, look around and compare quality to price (and engine to engine).
Learn about what you
CAN get for your money before parting with it.
.