None are perfect, but they would all work.
Personally I wouldn't go for the middle one - the panel van. I would say two annoyances with them... 1) they are pretty narrow compared to a van like a Sevel (Ducato/Relay/Boxer) and 2) the sides close in quite a bit as you get to the roof, which exacerbates the narrowness.
I went from a VW LT, which is same body as a Sprinter Mk1 (T1N) to a motorhome and the difference in the feeling of roominess was amazing.
I think I would prefer the low-loader body over the on-chassis box as you get the smoother profile with the luton, but having said that, you don't have the wheel well intrusion and you could make up underbody storage for fresh and waste tanks, plus LPG tank and Batteries, which would maximise your living space.
Having said all that, I am starting to prefer the on-chassis box now

It is not as high as my current Autotrail and you can get a cab aero-top add-on I believe.
The one for £6,500 (+VAT) looks like a good base to convert unless you want to do loads of miles and then better to go lower mileage.
Not really minded to do a conversion myself as happy with what I have, but if I were wanting to, I'd certainly consider that one
