hi Steve
I don't think you can do the odd carpet clean with just any machine,that is recommended ( its a bit like asking what tools should I get to change a clutch when your not a mechanic) , its training and the right solutions for the carpet that you are cleaning that is jut a important .
so training would be a good start then they would recommend which they thought was best for your needs.
Then with with the appropriate training you could make an informed decision on which machine and solutions to purchase .
And in the mean time sub the work out to a pro