I personally think t4 is probably the useable affordable range now, even the T25 is getting a desireable vehicle as the bays and splits get expensive.
The T25 is growing in popularity and value, cheap ones will often be rust buckets from my experience. They will also be more prone to breakdowns and need more TLC. A hands on approach is a must with these old buses, old classics need a lot of love and patience.
For the T25:
Bodywork is the primary concern!!! Engines are kind of a replaceable item at 30+ years old.
Pre 84 and earlier (air cooled) vans are generally considered to be poorer quality with regards to panels than 85 onwards (around 84/85 they switched to german factories), the engines also went water cooled around 85. 1.6d will go forever, do between 35 and 40mpg but will be very slow and struggle not to lose sped up hills, may frustrate someone used to a modern car. Petrol 1.9 are pretty good will do around 25-28mpg but . There are also 1.9d/td and 1.6td which are good engines if looked after.
Take a magnet to check for filler it may have a lot.
Check rear wheel arches they are the primary location for severe ROT.
Front steps and arches are prone to rotting, not hard to replace but will need a good welder.
Check the trailing arms at rear, expensive to change but new parts are available.
Front bumper is a water trap and difficult to get at to weld, check for filler.
Best place for T25 is club8090 wiki online, full list of common faults and areas of concern.