Check the clock, look at pedals see if you think wear match, open all doors, check every pillars for signs of rust or repair, open bonnet check closely around all connections, check in and around the battery and tray for signs of rust/repair. Look at all connections look for anything untoward.
If the engine is warm, don't buy. Start engine from cold, let run until warm, listen for knocking noises, take van for a good run not just around the block