In the interests of our fellow Mikeysboarders, I'm answering the last question first, and will answer the other two (sort of) separately.
Buying a used truck mount is much like buying a used car. You need to know "how used" it really is, have the equipment (and van if there is a van tied into the purchase) checked out thoroughly by a qualified and impartial mechanic.
The choice between a used van powered unit or used slide in starts with the same questions you'd ask yourself about a new van powered unit or slide in. Those questions have been asked and answered here since MB began. In the end, it becomes a question of what is right for you.
Assuming you are 100% neutral on that decision (and I'm not sure anyone can be...but assuming..):
When purchasing a used van powered unit, you first need to have the van checked carefully, as the cost of moving the unit to another van later is so expensive to be barely worth doing. If the body and drive train of the van are in decent shape, the biggest expense I worry about with van powered units are the recovery tanks. Make sure they have been kept clean and won't need major repair or expensive replacement. Water, vacuum, and chemical systems aren't much different than slide in systems. The heating system is simpler in most cases.
Used slide ins have the distinct advantage of being available separate from the van if you wish. In that way you aren't stuck with accepting something less than what is ideal in one or the other because of how the van powered units are so tied together. Used slide ins usually have more complex heating systems (the vac, pump, and chemical systems being nearly identical) and that part is more difficult to find someone able to inspect and give you a fair condition report. In the case of a used slide in, are you looking at something being used by a fellow cleaner, or one that has been taken on trade and been refurbished like John or I or our new friends from the Cleaners Depot might do?
How much actually distributor does with such a machine is something that will require a good level of trust on your part. There are units that are just taken in, cleaned up, and anything that wasn't working was fixed just like a standard repair job. Then there are units that are stripped down with many components rebuilt or replaced. Its important to know the difference.
If you get a used slide in, I'd make sure it can be serviced by the closest distributor to you, and perhaps if there is a warranty involved, even purchase it from that company.
We have so many people here on this board who have kept old...nearly ancient..machines running that once you see something you might like to buy, I'd ask here about long term durability of such units when purchased used, or after they accumulate a lot of hours.
