Hi!
The main things are the usual thing I think, you may wish to look in the Tech section:
https://hiace-super-custom.forumotion.com/f1-grease-monkey-sorry-couldnt-resist-its-a-bf2142-thingWhich camper conversion is it? There is a very good 'Cruising Cabin' factory conversion I'd recommend over all others.
I'd look at rust first, general condition and evidence of servicing.
The left hand seat is the bonnet, it unclips so you can see the engine and hooks to a strap on the door pillar.
If an auto, right at the back under a clip is a red dipstick, have a white tissue ready. Take it out (about 24" long) and clean it, pop it back in and out - then wipe on the white tissue. Red is good. Brown needs an oil change. Black: walk away.
Drive it, it should drive well and note the gear changes are clean and natural.
Check inside the radiator cap for clean anti-freeze and inside the oil filler on top of the rocker cover for clean oil. Oil itself on the dipstick will be black in a diesel but still look like oil!
Also check the braking is smooth and confident: the front discs are worked quite hard and can 'warp' - and require the hubs popping off to change.
(See
https://hiace-super-custom.forumotion.com/t3149-torque-wrench-figures-for-2wd-super-custom-front-brakeshubs#16875 )
Remember they all start out perfect at the factory, Toyota is top quality, but are still subject to lack of servicing and abuse.