Two threads on Vw Bus related matters in a single week?! Wow.
Great vehicle to do a restoration job on, but it is really important that you know what your letting yourself in for. Also, after a bit of time around these cars, you may decide that what you thought you wanted originally (for example, a complete resto camper), isn't what you really want. So make sure you've decided what you want from your VW before you begin to restore it.
I wouldn't get one from America, but that's only because I want RHD - this may not bother you. Check the ID of Buses carefully and paperwork still counts for a lot. Don't be too worried about different engine numbers though, Buses eat through a few in their life. You'd also be suprised how many Buses have been tampered with, different ID plates etc... be careful!
There are some great websites out there that will give you some information. Below are a few that sell VW Buses (they are garages / traders) that might give you some idea.
VW Relics
FBI VW
Kieft & Klok - Not UK, but some good Buses
My ideal Bus to start on would be a completely solid, but pretty tired '68 Bay. I wouldn't go near a Bus that has such awful bodywork as the amount of structural repairs are huge, expensive and time consuming. There are some great, original, honest Buses about, I come by a few every few months, but there are tonns of rotters too.
Stay away from early '72 and mid '73. Due to the being part of the 'cross over' years, they have a strange habit to rust very badly, a lot worse than other buses. In a resotration this can be sorted, but it's not worth it imo. If you do want to keep it all stock, go for a '71 Bay. It's got the good early looks of a Bay, slighly better engine and front disc brakes.
Finally, I wouldn't go for an already resotred Bus, as these vehicles are so flexable, it's nice to put your own spin on them and learn a lot while you do it. Hope this helps a bit.