Jason,
I have a Max 450D which we use as a standby unit to our Titan 875 when cleaning cruise ships. We bought the unit second hand and it had quite some hours on it but it performers perfectly. The solution temps it produces are fantastic and of course it is a dual operator unit so you can get a lot of carpet cleaning done very quickly.
There is very little that can go wrong and as it is diesel the engine will run for ever.
If you're going to look at one let me know and I'll tell you what to look out for. Better still, take someone with you who knows about TM's, there plenty of them on here.
Simon