6hp would barely blast fresh muck off a car (well that's maybe exaggerated
) Have a look on ebay, theres a company called Jetmac, I bought mine off them last year, 3000psi Honda 13hp, does a great job, theres a bit of movement on the price if you haggle a bit.
They stock all the parts you'll need and can source you a flat surface cleaner as well if needed.
To be honest winters not a great time to start, the spring/summer was bad enough with all the rain.
It's pretty straight forward to do, so don't allow yourself to be ripped off or sold products you don't need.
A 13 hp Pressurre washer, turbo nozzle should be good enough to start with if you want to keep costs down, and when you get a bigger job buy the FSC and the job will pay for it.
You could expect to spend around £800 - 900 on a decent pressure washer, £50+ on a decent turbo nozzle and around £500 on a stainless steel Mosmatic FSC.
Thats all quality kit and with a bit of luck it shouldnt take long before you recover the outlay on it.
I think in general it's been kinda quiet for most people this year so think carefully before commiting yourself. It might be disheartening starting out at the quietest time of the year.