I'm at the early stages of looking into something similar. There's a lot of information about rear seat conversions around via Google.
As a T4 owner I guess you already use vwt4forum.co.uk there is also a decent looking forum at caddy2k.com but they have some of those minimum posting to get access to all areas things going on which is a pin.
In short there are steel plates over the footwells that need to be removed and then some fairly simple loooking welding (if you're that way inclined) and fixing required to put the seat mounts in.
There seems to be a bit less information regarsding insurance implications but from what I have seen people just advise thei insurers that their van is a 5 seater. Dunno about V5 implications though.
It's enough to make me want to spend a bit more time looking for a van that's come 5 seated from the factory.
It seems to be that it's the next van down the line after the T4/T5 in terms of nichdesirabilityness