Having spent the last year to two years on and off looking for a good van....
i wouldnt worry about engine and transmisions etc..... as stated the chassis is the most important things, so many are so bodged and patched and schutlz to death. Also look carefull as people do patch them and use filler to ''BLEND'' the repairs and chassis can look good but isnt.
Then you want to have a good look at the roof, gutters inside and out, also see how the back doors fit and check the panel below the back door esp the corners, this is a two panel item, you have a inner rear valance what bolts to the chassis and then the outer. Also back light areas rot out as collect mud of the back wheel. also check outer rear wheel arches outside as these do rot through. rear floors go along the outer edge near the screw down wooded floor, these rear floors are a box section floor and difficult to repair unless you buy the correct items, theres a company called fairmille restorations, ian mackenzie who makes most if not all the reapir panels you would need. Onto the cab- well these can and do rot badly, check cab gutters, b post bottoms, floors and sills, lift the seats forward and check the corners, this area is normaly well plated up and bodged. lift the cab doors up and down and look for movement in the a pillar. inner wings go around bump stops, bottom corner and flitch area as most minors do and also check the tie plates ( engine bay floor either side of the engine ) .. vans also when left the factory didnt have chrome bumpers and overiders and different bonnet and badges, but thats just small detail.
I wish you look in your search. You get what you pay for im afraid. Theres alot of bodged to mot standard vans out there and youll be paying a good few pounds for a good one.
To summarise chassis is the most important area, then the van back, as the cab is pretty much 4 door saloon and repairs are more common and reapir panels etc more available.
Price wise, expect a full project for anythin less then £2000, £2000 to £4000 you should get a useable van, and £4000 to £10,000 for a minter.....
If i was you id buy the best you can and spend around £5000 to £7000 if you can budget that, the cheaper vans will never be worthwhile in the long run and will become money pits and to do the work right you have to be skilled or pay someone which again is expensive in the long run. Id also recomend getting what may look a tatty van but mint underneath, ie someones just used it but oiled it underneath etc, there the ones to get, good clean up and respray and maybe new wings, door repairs and odd bits of welding a respray and you got a good one.
Also follow your head not your heart, dont get fed up keep looking, one will turn up. Never make do as you will never be happy. Ive just bought a van and its a cracker, never been welded chassis with mint roof etc and doors shut like new, but ive looked at must be 30 vans till i found this and i paid £5500 and im repainting it now. They are rare but are out there....!!!!!!
Good luck and any help us mmoc member are here
