You don't have to mount the machine permantly in the van if you don't want too. as Rob says, it's all about the reels, you could just mount a large hose reel in the van and work from that, that way if you ever need to take the machine off you just uncouple the reel and take the machine off.
when I first started I used to work this way, and even just has a wheelie bin in the back of the van set up with a ball cock, then as time went on I got got larger tank etc. to be honest I have found that the best size tank for me is a 550 litre, this gives me just short of an hour (on and off the trigger) but yet also leaves lots of space for other things
However most my work is domestic and drains at the minute