You will be riddled with problems if you but a vivaro/traffic/primastar for under £6500, and below 52 and reasonable millage,
personally i would recommend, and ex BT van, if you get it around 70,000 miles it will still have loads of life left, BT really look after their vans and it shows,
They have a 1200kg payload, programmable eberscaper heater in the back, hot water heater, safety deposit box,
If you dont want any of these things you can sell them for near £300, and there is also the racking, which is secured to the chassis, so makes mounting a tank super easy,
You could make your self a diy system you know for half of what you are going to pay, and despite what people tell you ratchet straps secured to the chassis via a steel frame is more than enough
oh yeah its got a bulk head too
If your tight for money then just source all the parts from ebay and just spend a few days trailing through goggle etc etc.
Oh btw how many clients do you have