I started off 17 years ago with a Numatic CTD575 - they still do a similar machine for about the same price - approx £600.
Twin vac motors, about 45psi.
Worked it for many years but only doing the odd carpet clean.
It will do a job if YOU put the effort in.
Obviously now cleaning fluids are better then 17 years ago.
What I would do now is get a good non detergent cleaner ie M-Power/Nemesis, DFC210, Pure Clean to use as a hot prespray (not too much), get a good stiff long handled brush and give it some good agitation (a proper workout), and then rinse with clean water or with a bit of M-Power/Nemesis.
I reckon you will get some good results. You will also need some spotters.
Dont be tempted to use strong detergents. Yes it will clean the carpets but you wont be able to rinse sufficient to get all residue out, therefore attracting dirt quickly.
Use any profits to save up to buy a bigger more powerful machine.