Well I'd better support the other side - i.e. hot water extraction.
You're already doing domestics, and I honestly believe (I know) that HWE is the most thorough clean you can do, and domestics is all about getting the price per job as high as possible, by putting quality (perhaps perceived quality) first.
All makes of porty are so similar it hardly makes a difference - I'd go for a good twin (or triple) vac, second hand. You want a heater, despite what everyone on clean talk says, and you want about 300 - 500 psi on the solution pump, with no water flow. By good, I mean one that looks right, from a person you like, with a good story.
You should be able to get one for £1000 to £2000 - maybe including other gear, like hand tools, snail drier, etc., then use the rest on marketing.
Low moisture is very good for a lot of commercial, and a few people persuade domestic customers to use it, but it's a hard sell.