You'd probably be better off looking at a motor home, ready installed water, air-con, etc. you'd just need to remove seating and install your treatment couch. They are also fitted with 'steadies' - wind down feet to steady the van whilst you move around.
You will probably have to take your extra driving test, but it's not hugely expensive, I think it's a couple of hundred pounds, which you should be able to offset against your business costs, or you'll end up with a vehicle too small for your needs and it will become uncomfortable for both you and the client. It's a lot of time and money to invest to get wrong.
I think your biggest problem will be insuring the vehicle, it's unusual, it will have quite significant modifications and will be a commericial vehicle so will immediately be higher premiums. About 10 years ago I had a little Peugeot 206 van which I had fully signwritten for my business. In the second year my insurance renewal came in at over £800. When I queried it, I was told a commercial vehicle was subject to higher premiums despite the fact that I didn't actually use it for work, I'd given up mobile I just used it as my day to day car. If it had the car, not be the van version (no seats in the back) my insurance would have been £300 even with sign writing for the business. Madness but you can't fight insurance companies.
Did some googling, found this too which is what you're trying to achieve I assume
http://www.beautyonwheels.org.uk/mobile-beauty-salon-bolton-mobile-beautician.html
Note: the VW Crafter is in the 3.5 tonne category
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