Hi Clee,
It depends on your market. If it residential I think you could easily get a used Prochem machine.
Especially if its the wife doing the work. You will need to consider the weight of the machine if it is your wife doing the lifting in and out of the van or car with the machine.
The machines are fairly bulky and heavy. Inspect one or two before you buy. Habe a look on ebay you do get some good deals.
Budget for at least £1300 on used equipment and you will have plenty. Have a look at some companys selling used carpet cleaning machines.
Also consider about repairs, so distance to and from the repairer or shop is fairly important.
Before you purchase a machine, consider these points,
Spares available
Weight
Power PSI
Hose length can it handle. The larger machines like Ninja, Airflex turbos can handle a greater amount.
Competition
Water filling and emptying
Will you need to lift the machine upstairs in customers house as hose length poor
Dont forget if you need to llift the machine upstairs its not professional, and you could damage walls, banisters etc.
Check out clean smart supplies as they have now made several airflex models they maybe suitable for your wife to lift and also be adequate in power to give a good result.
Dont rush and buy one, think carefully first.
Surrey,
The only bit of strength required is to lift a portable machine in and out of the van. - We have ramps. Saves the lifting
Most portys have wheels on that make the machines easily moveable.
We have a few places that its a two man job as you have to lift the machines up stairs for access to the building.
Dave