With any van you need to see what it is like underneath. They aren't really built to last decades and 5-10 years of running on UK wet and salted roads can wreck the chassis and body very quickly.

My 2005 VW Transporter with 182,000 miles covered is in lovely condition both body and underneath.

Isn't some of the information you want, Emma, given on this link?

vanguide.co.uk/ford-transit-dimensions

As far as I am concerned, if you plan to spend a lot of time, money and effort converting a van into a camper which you plan to keep at least 7 years, what you need to focus on above all else, is to get absolutely the best base van funds will allow. And, IMO, a 120k+ mile Transit is not that.

Whether or not a Transit would be the best choice even with lower miles is debatable, though given you already seem to have decided that it must be a Transit L3 H3, it is a moot point.

You have not said what budget you are working to, but my advice would be to forget about the 'full conversion' at this point in time. Take most of what you are budgeting to convert the van and add it to what you plan to pay for the base van. The only thing you really need is to insulate it and line it, most everything else can be done in due course as time and funds permit. A camping stove, a couple of inflatable mattress, a couple of beanbags to sit on are all you need to get going. I'd also question whether or not you need such a large base van for just you and your son. A smaller van is not necessarily going to be cheaper to buy, but it will be easier to park, cheaper to insulate and line, and probably more efficient. Years ago my parents had a camper which my Dad converted from a VW T25 van, not very big at all, no high roof or anything, very basic inside. The only thing he'd do differently was to have it insulated properly and have a pop up roof so they could stand up in it.

I think you have sound advice from bbd and slo here. I've been toying with the idea for a couple of years but a smaller daily driver day van sort of thing. The used market is up about 25% so it's a bad time, though Jan might be as good as it gets at the moment. All the decent advice is to buy as new a van as possible and go from there. Look at ducatos too, all the bespoke stuff is based on that. You may not need h3, you can add a higher top bubble to it when you know you actually need it. About £2 to 3k. Remember a big van will only give you 25 to 30mpg, significant if you do a lot of miles. I wouldn't buy a cheap van and spend thousands on the conversion, you'll lose that value straight away, so keep it as new and simple as possible to start with. It's about what you do and where you go rather than the fanciness of the interior. An awning or tent will add loads of practically no n summer too. Best regards

Incidentally, the FWD Transit has an extra 10cm interior height over the equivalent RWD version.