Depends whether you want to use it for transporting bikes or use it when you go biking, ie to live out of. Once you’ve worked that out you want to work out what the biggest van you’re happy driving around is, and what options that leaves you. A T5 is quite a small van and ideal for carting bikes around, but you might want something bigger if you want to live out of it and definitely if you want to sleep in it.
How many bikes do you want to transport and do you want to be able to just chuck them in or are you happy to take wheels off and have them take up less room?
I want to do a Transit LWB medium roof type van with three seats up front, three in the back facing the rear, with a table. Bikes suspended from the ceiling at the back without wheels and bars turned, then storage for a lot of stuff everywhere else, but nothing down to the floor, so there’s room to put a mattress in. The idea would be to carry a tent usually, but would want to retain as many options as possible.
From there you can start adding as much or as little stuff as you like, depending on budget and use etc.