Surely the best thing to do would be to get a van that's right for the job, you can always extend your garage upwards

Seriously, if you're going to commit to such a major investment as a TM then don't handicap yourself from the outset. Even if the machine fitted in the small van and even if it wasn't overweight, it'll drive like a canal boat. This is a very serious safety concern, what's going to happen when you inevitably have to do an emergency stop? Ideally you really want a 3.5 tonne van for any TM regardless of size, because of the water.
Either that or you have to arrive with an empty tank everywhere and pray you can connect on to the water supply... in my experience that would leave me shafted 50% of the time.
Also you'll find that you'll have no space to carry all those little tools and nik-naks you used to carry on your old van, you'll be forever taking them off and putting them on depending on what jobs you're going out to that day.
I bought a MWB med roof transit for my TM installation (blue wave), I thought I'd have acres of room but it's still a bit of a squeeze even with some cunning organisation in the back.