If you are just going to service your existing database of cleaning customers then you will get away with a small budget of £600-£1000. Thats how I got into carpet cleaning and now I have a truckmount and the most powerful portable machine on the market as a backup.
However, I started off with something far removed from them - and still use it 18 years on, albeit it is now a spotting machine - used this Monday to remove red wine spill.
The is a Numatic machine, very similar to this in spec
http://www.totalcleaningsupplies.co.uk/numatic-ctd572-carpet-cleaning-machine-83-p.aspI am not promoting the selling company - never heard of them, just google to get the detail for you of the machine.
You will find it hard work as the machine iitself is a base machine and not all that powerful, although used correctly (and thats up to the operator) can do a good job - it will take time to do a good job though.
A more powerful machine could do the same job in half the time - but thats a different budget.
Biggest thing to remember with the less powerful machines is not to put too much water down as there is not the power to get it up again if you saturate the carpet - wet carpet even the next day, not good.
You will need a good UPRIGHT vac with a powered brush, your Henry is good - but not for use before carpet cleaning. Sebo BS36 or get a 2nd hand Kirby.
My database of regular domestic cleans (yes I still do it) gets a reminder every month of my carpet cleaning service - but very few make any sort of enquiry - so dont expect a big take up. And if you want to expand on more then one carpet clean job a week I would recomend a much bigger budget.
Hope all that helps.