Check everything, alot of grumlins can be hidden under conversions.
Also check out: http://www.vwt4forum.co.uk/
Currently converting the company T4 (2.4 non turbo, shortnose) to a do everything, van, camper, dayvan with removeable kitchen, seats, beds etc.
Out of all the vans I’ve had (vivaro, transit,(although sprinters are nice for a bigger van although rust a bit))
I like the T4 the best as it’s easyily the best put together and very reliable it look after properly.
121K is reasonably low for the 2.5 turbo engine, as long as it’s been serviced on the button and not been fiddled with by a bodger.