It's a good idea to go out with a local cleaner first, no matter what equipment he has.
It is to learn procedure, a bit of chemistry, different types of carpets, how to handle customers, etc.
Go for free- like an intern, if necessary, at no pay, just for the learning.
Or, go to work for him as an employee to learn the business, like everything, it looks easy because the experts have spent a lot of time and money learning what they need to know.
Leaping into the business without knowing anything is sure doom.
The first carpet to clean is your own, then friends and relatives.
You will be using muscles not used for other purposes, and can be a big strain on arms and back.
Or it may turn out you are allergic to some chemicals. There is no way to know this in advance.
Lifting a portable or other machine in and out of the truck can be tough, so before buying any machine, lift it, and see how heavy and awkward it is, and remember you will do this hundreds of times. Some machines are lighter, or better shaped and are easier to deal with than some others. And the same in homes, up and down stairs can get tough- more than in and out of the truck.
Anyway, we are here to help all we can, since we all have been there at one time or another.
Gary