I bought a karcher 601 diesel steam cleaner with the same intention, it ended up being used for cleaning my truck....... If i was serious i would try and buy a new or nearly new one which will set you back at least £1000, fibreglass the back of your van, water tank ect. better to be self sufficient.
With such a business advertising is the key to earning a living...And reasonable prices too...
Work wise think about 4x4s diggers lorry's caravans boats driveways ect so get on to hire company's holiday parks and even the council for bins and chewing gum removal.
And make sure the machine