David,
We wear the hats of both carpet cleaners and window cleaners (and also contract cleaners), but I went in the other direction to the one you're taking. I started off carpet cleaning then moved into window cleaning.
Carpet cleaning is much more specialised than window cleaning, it also requires a greater initial outlay for both machinery and insurance.
If you fail to clean a window correctly, all you end up with is streaks, however, if out fail to clean a carpet correctly you may end up with a bill of £,000s.
If you are also going to undertake upholstery cleaning, then this is even more specialised and prone to disaster.
There is a vast variety of equipment, chemicals and training out there to choose from, you need to look into all the options closely before deciding which route to take.
Spend some time on this side of the forum and you will get a good idea of what people are doing and the pitfalls that can happen.
Ideally you want to get out to the manufacturers to see what they have to offer, the ideal place for this is somewhere like the NEC cleaning show, which I think is next March.