Lee
A truckmount is the best tool in the box.
But before you choose a machine, first you need to see what your needs are. If you haven't attended a formal training course yet, then do that first. IICRC and NCCA are the leaders for generic training and Prochem, amongst others, are leading supplier/manufacturer trainers.
If your intention is to clean carpets as an aside to your main core window cleaning, and you are looking at maybe a lounge or two a day, then a quality portable would probably be the better business decision. If you intend to clean very high rise buildings, office carpets, or high security premises where the doors and windows are locked all the time, you may need to look at a low moisture system. If you are looking for high productivity and high volume, especially with high soil loadings, where access is not a problem, then a Truckmount is the way to go.
Speaking as someone who doesn't run a truckmount, the impression I get about running costs is that you are looking at between 40 and 80 hours running time between services plus the cost of the fuel. Initially this may appear to be a heavy overhead, but because of the immense productivity rate, the cost is a very low price to pay, and regular service is easily completed quickly by anyone who knows how to hold a spanner.
As for purchase price, it would only be fair to give examples of new prices and then you can place your own values on previously owned machines and van.
Most of our industry suppliers and manufacturers will now offer a package of truckmount and van along with the necessary options and upgrades. I would estimate that a quality new entry level T/M and van setup would cost in the region of £25k. There are other lower cost options available that don't follow the traditional T/M configuration about which we're hearing good reports. I'm thinking along the lines of the likes of the Prowler and the Phoenix.
Safe and happy cleaning
The Ken