Welcome Mitchell!
If you're getting anything older than 2000 (i.e. smiley face mk5 or older), which for £2,500 is probably what you'll be looking at, look out for oil on the rear diff, the seals are often a weak point and if not replaced correctly cause an axle whine. Proper repair is expensive. Also these vans are notorous for rusting wheelarches and door sills (my 2000 V reg already has started!) - so check these areas and be wary of very recent resprays, they may be hiding something. Make sure you also check the heater switches - the resistors in these often break, particularly on the fan speed switch.
If you're lucky enough to find a mk6 for your budget (new shape Transit) make sure the gear selection is smooth in all gears as the selectors are a weak point and that would lock you in 1st/2nd, 3rd/4th or 5th/Reverse only.
Otherwise it's just checking the usual, electrics working, knocks, rattles, HPI, service history etc. You'd be best getting a turbo diesel to cope with the weight of the kit and the long journeys.
If you are doing a conversion yourself, look to get at least a semi-high top, although if you're going abroad, a high top or even a high-cube (like Securicor Omegas) would be best for standing room. There are several topics on here on conversions, and one at least I know of with photos.
http://fordtransit.org/forum/viewtopic.php?f=27&t=12004&p=106967&hilit=camper+conversion+photos#p106967If you want windows, you might want to consider a minibus for conversion.
One thing's for sure the Transit is the best option, they're so cheap to repair and parts are always available.
Hope this helps!
Chris