I'm a truck mount distributor, so I can't tell you that I'm being entirely objective in my comments.
With that said, we have found the Apex to be one of the most trouble free machines that we've sold. As Dave said, hours alone aren't enough to say whether or not its something you want to buy.
I worry when a guage needs to be replaced, because that is often a sign of freeze damage. If you aren't familiar with truck mount machines, I'd recommend that you have a regional distributor check the machine out and assess its condition for you first. They might want to steer you toward buying a new one from them, but its worth that discussion to get them to look at it as a third party.
If not, have a Kubota dealer look at the engine, then make sure that it produces pressure and that the pressure can be adjusted, that there are no leaks anywhere, that the chemical injection works, and that it comes to the proper heat and does not overheat and shut off. Also see to it that the vacuum is working properly, not just by feeling it at the console, but also by running at least 100' of hose on it and seeing how it cleans and dries carpet.
I would recommend going out on a job with him if its still installed in a truck, or doing a job of yours with it if its convenient.
And I would not buy it without him having him replace the pressure guage so that you know the machine is working properly.