No advice here about buy this or that, ultimately you'll get what's available and what you can afford, but if you get a choice then these are some points to think about.
This determines just how big it can be because if you haven't got space then it will be a pain. No good parking it three streets away if you have to keep carrying gear to and from it, and the conversion will take for ever. Also the size can annoy neighbours or there may even be a permissions issue where the vehicle will be stored, or just plain prejudice. Figure these things out early on and avoid the pain of a bad purchase.
Regulations change from year to year so finding that you need to pass another driving test just to get your new vehicle home can be disappointing. Not only that, many a vehicle has been built for someone tall and strong, having to get off your seat to stamp on the clutch wouldn't be fun and if you only have a licence to drive an automatic then that will limit your choices.
Maybe the reverse thinking but while parking a car on your drive is easy, that 5T long wheel base van may not be, and can you walk past it afterwards when you do. Where do you want to take it to, there are may destinations with height restrictions and ferries often have similar tariff changes based on height. So be prepared to think about how small you can get away with, as well as is it big enough which is the other question.
A small vehicle can be enlarged with awnings, but if it's all you have to live in on a cold and wet holiday you may regret having a compact solution.
Choosing a vehicle that will be easy to convert is important. Easy may mean you know someone who can do it for you, but if it's only you, then there may be more work than you want. Larger vans will have lots of working space, and be easy to work on, smaller vans not so easy. But a larger van will be a lot more room to fill, and will be more work. Do you need the space. Does the vehicle have really good looking lines that you will have to mould fittings round, or dull straight sides that are easy to fit to.
Does it already come with windows you can use, will you be able to fit new windows where you want to, and if they aren't what you want, how will you change them?
Do look underneath the vehicle to see what you can attached and whether you can work underneath it ok. Finding the fuel tank just where you wanted the waste water tank will give you issues.