Transits rock! I had one for years, went round Europe loads I still miss it.
Random stuff I remmember:
Take care of the door hinges/ doors at the back, they are prone to rust + expensive/pain in the @rse to replace. Speaking of doors, fitting some extra security is a good move Transits aren't the most secure vans in the world.
When boarding out, used some super durable plastic washers between the metal and the wood (scavenged from an engineering shop). This reduced creaking and noise a fair bit. Didn't fit side windows, so could park up anywhere + mostly didn't get any hassle. Just looked like a normal van from the outside. Not very cool, but having Swiss border guards and french coppers giving your an internal inspection isn't very cool either.
We used the van for MTB-ing, and touring with bands so fitted a second row of seats we got from a breakers yard + also bolted in seatbelts (lap belts) gathered at the same time. We had to get a mechanic to check these were bolted into the cross beams, not just into the metal. The insurance Co were a bit funny about "modifications" but we just said it was a professionally done camper conversion sent the mechanics report, MOT + they were OK in the end.
Some insulation makes a BIG difference in winter as does having some ventilation.
Didn't bother with a burner, sink, cooker as we needed that space to be adapable. So built a ply storage unit, that had a place where you could put a camping stove, fit a bowl on top, storage room in it for, water, food, beer, a gas bottle and spare battery for running extra lights, stereo e.t.c. Had a collapsable bench/fold-out bed on the other side with storage underneath.
I think the most important thing was being honest about how the van was going to be used for most of the time. We needed an adaptable van. if you're going to be living in it with 2-3 people for weeks at a time youre going to need it to be comfy as.
Good luck with it, its great just thinking sod-it We're off for the WE/Week.