Prochem's Steampro (120psi and two vacs) is adequate for start up. Brand new I think they are £2000 plus VAT, but you can find them on ebay second hand for anywhere between £600 - £1000+. Don't be tempted to buy something on its last legs though.
Or even a Ashby Ninja around the same spec.
Some people are even selling whole set ups on ebay! A lot of people thinking carpet cleaning is easy ha ha.
I think Doug meant that you'd need 10k as "dip into" fund for the days/weeks you have no work. The first couple of yers will be the hardest. You have a van already so that's a good start.
I heard that oven cleaning is a good add on service to carpet cleaning. Its hard work and the pay isn't as much as cc'ing but a lot of work out there and probably more frequent in the early days. From what I've heard the going rate is around £50 and the average oven will take 2 hours. Better than sitting at home watching the telephone.
I would consider window
licking cleaning if I where you though. A lot easier to set up, cheaper to run and operate, and you can easily train a monkey to do the cleaning for you while you drum up the business once you get to that stage.
Take a look at the window cleaning section. They are all inbred simpletons there, how hard can it be!