Richy, we clean carpets. We do alot of it and it's not really an add on mate. If you make mistakes, it gets expensive.
If you look at the topics on the carpet section, you will see how much you need to know. A decent machine will set you back a few K and that's for a portable. It's not like Trad or WFP. There is alot more to know. If it was so easy, any plonker could buy a Rug Doctor and a bottle of carpet destroyer like VANISH and earn money at it.
If you go into it, don't poke it with a stick mate, it's got no real room for trial and error. See if you can find a cleaner to take you with them and see how they work and how to save time , money and aggro.
I love it, but love upholstery cleaning even more. Good turn over and interesting work. If you can find enough work and learn what you need to know, sky is the limit.
Best spend some more time on the carpet section mate.