Hi Mark,
I've been using a prochem steempro with a 70psi pump and single vac for nearly 2 years now. Its done me well but very underpowered. I've just bought a ninja with 135psi pump and twin stage 3 vacs and the dfference in cleaning and drying times are very noticeable.
Before I was using a single jet wand covering 4" to 5" of carpet at a time often needing more than one pass to clean and vac. Now I'm using a 2 or 3 jet wand covering 10" to 12" at a time and only requiring a single pass.
I'd go for at least 100psi with twin vacs, my prochem machine has been 100% reliable but cant comment on the ninja yet as I've only had it a few weeks but has been outstanding so far.
Don't make the mistake I first made, get a better machine and stick with your van. You can have the biggest, best and newest van in the world but if your machine is under-powered and leaving customers carpets / upholstery unclean and soaking wet then you will get no referals or repeat business which is the best type of business you can get.
Hope this helps.
Kind ragards,
Jason.