Adam, I think David is right you do need a good reliable machine, but there are good second hand machines out there you just need a little luck and keep looking around.
The nature of carpet cleaning means there are a number of people who fancy getting away from a 9-5, get a new machine but cant make it work so end up selling there hardly used machine, usually on ebay.
Thats how i got my Commanche, used 6 times and in as new condition for only £800

Paul