The (T25/T3) Multivan seats are probably the easiest to do as you may already have the brackets in the floor for the bottom bolts.
There were two types of seat; one that used retaining pins to hold the back rest and one that doesnt - The one that doesn't would make life easier to install as it only relies on the floor and bulkhead fixings to work properly, where as the other would require welding of the retainers to the side panels.
Below is photos of the two types for reference.
With retaining pins:
Without (The side panels have ears moulded in them in this photo, but they don't support anything):
Other options are: Standard; Caravelle rear seat and Bongo rear seat with sliders which I had looked at doing on my previous T3.
Whatever seat you use, you would need to use the factory seatbelt fixings on the upper rear quarter panels and you may find that the rear bulkhead is already fitted with the captive nuts to mount the lap straps.
J.
Would I be able to fit the rear seat from a t4 caravelle into my t25 without too much hassle? I like the fact that the seat belts are mounted onto the seat itself, or would you go for something like a bluestar seat? 
