Hi,
It will likely depend on the type of carpet you are cleaning predominately to which machine, method and solutions will benefit you the most. If you are working predominately in commercial cleaning am I right in thinking you are cleaning carpet tile and low profile synthetics the most? If thats the case going down the encapsulation route is probably the the most cost effective and profitable way. You will need a good upright vac, a cimex, orbital or rotary machine. The cimex is probably the quickest. You will also need brushes, pads and bonnets depending on the machine and surface a sprayer and a selection of good encapsulation products. All of this can be bought from Restoremate. You will be looking at about £3000 to get going. You can start encapping on a shoe string though. My slow speed rotary machine cost me £80 off of ebay and is bomb proof. So a second hand set up with a vac and chems could set you back less than £400- £500. It is also wise to get a small extraction machine as well though for stains that really need extracting.
If you would like to go down the Hot Water Extraction route there is lots of information on this forum if you use the search bar.
When it comes to courses Prochem are pretty good however there are courses out there which cover just commercial carpet cleaning if that is your area such as this IRCRC course from Alltec
https://alltec.co.uk/training/iicrc-commercial-carpet-care/If you would like to become a "complete" carpet cleaner who covers all carpet types in a variety of setting It will be necessary to have all the cleaning methods at your disposal. It is also wise to do three courses, Carpet cleaning, Upholstery cleaning and Spot and stain removal. These courses can be done at most of the main manufacturers and sometimes a course comes free when you buy a machine. I have personally attended courses at Prochem, Ashbys and Cleansmart and they were all good.
All the best wit your venture.