In reply to 1poundSOCKS:
We converted a SWB Vivaro last year (in hindsight a transit would have been better!) Would recommend getting someone external to fit the windows. I did one side, but wasn't worth the hassle/time against how much it cost to get a window fitted (think it was about £150) Roof vents are pretty easy to do thought, just worth banging some extra sealant on the roof after, even if fitted properly the seals tend to break down over time.
We are insured with Adrian Flux if that helps - they are pretty good really, just tell them about the modifications we do. Worth taking some pictures once it's all done and sending them off to the DVLA and asking for it to be re-reigstered as a motorhome, if you're lucky they'll write back and say no, but you can register it as a MPV, which seemed to help with insurance.
I'll pop some pictures up later if I remember.
Juan
P.S - not sure from your OP if you are thinking of getting one or both done. Would recommend getting the non door side first, as when you are in it and have the door open you don't lose your window as well. (hope that makes sense)
Post edited at 14:25