Business wise cleaning is cleaning (unless there is a specialist aspect) and so I very much doubt that you'll be able to set up seperate businesses. That said, why would you want to?
As a first machine I'd go a bit cheaper. For the market I'm guessing you're aiming at then 15 lpm / 200 bar will be plenty. Speak with Malcolm at Malcleanse, he'll point you in the right direction.
Fuel wise, it's just a matter of working out your costings againt proposed usage. Higher initial cost of diesel won't be recovered in a hurry if rarely used.
Have you considered a used machine? There are some great bargains to be had, especially this time of year.