As has already been said somewhere on the ambulance threads, check out the steering carefully, particularly the tie rod ends. Mileage is also important, but do you look for a low mileage unit (maybe lots of city driving?), or a higher mileage unit (highway driving?). There are arguments both ways, but I bought a low mileage unit that was used by a VFD in a small town - my 1999 Medic Master had only 52,000 miles when I bought it. I assumed that ambulance miles equaled hard miles, and so far, I've only had the usual running maintenance items to handle (brake lines, brake calipers).
Also, if the unit you look at has hydraulic brakes, and it's over about 10 years old, plan on replacing EVERY brake line soon. Ambulances live in nice, warm, fire houses, and they are usually parked over a floor drain, so the underside is constantly subjected to snow/slop and then warm and damp back int he house. The result is a short life for brake lines.
Virtually every ambulance out there leads a hard life - instant engine starts, full throttle starts from a stop, hard braking, and lots of time idling. The chassis are durable, as are the modules (the ambulance portion), but maintenance is definitely important.
Accordingly, I would tend to look for a unit sold by a volunteer fire department over a unit that was owned by a private ambulance company. FDs are used to preventive maintenance, and probably won't be as likely to let something slip until the next time the mechanic sees the unit. A municipal-owned unit is also usually a well maintained vehicle, but always check to see what has been removed to be saved as a spare part for a different unit in the fleet. Sirens, oxygen equipment and lights are often saved, but watch for someone who decided to save the inverter or electrical equipment. If the warning light lenses have been removed, you now have a hole in the side that you have to plug. Even though you can't use the old warning lights, the lenses can be painted to match the body, and left in place.
If the inverter has been removed, and just unplugged, it's not a disaster (except you have to buy an inverter). The Anderson plug on an ambulance inverter is easily purchased, and another inverter can be installed easily. Be careful if someone has just cut the wires - that may indicate that wholesale parts robbing happened.
If the unit has a digital control package for the module, check to see if it works before you plunk money down, Some of the components may have gone obsolete, and may be difficult and expensive to replace. If the unit uses the Weldon VMUX control package, the good news is that it's still made, but the bad news is that it gets expensive very fast. Even on ebay or used emergency equipment sites, plan on from $250-500 for control nodes (usually about 4 are used to do the actual switching), and up to about $1000 for a control module (control panel).
For those reasons, I bought a unit with conventional relay technology - if a relay dies, a new one is only $5 or so.
What brand to buy is also a big question - the decision is usually based on availability, but different makers use different components. I looked for a Freightliner-based unit with a Cummins engine, but I could have also had a Freightliner with a Caterpillar, a Mercedes, or a Detroit Diesel engine. Some brands have more headroom than others as well - my Medic Master has 72", some other makers have only 70".
I went with a Medic Master unit. Built by American LaFrance, Medic Master built a good unit, but they went out of business in 2008, and American LaFrance went out in 2013. Since ambulance users usually sell units that are over 10 years old, Medic Masters are seen fairly often on surplus sites, while other brands may be retained as spares by the users. Being made by a maker who's out of business also tends to reduce the price for used units.
Parts aren't a worry - parts for all ambulances come from the same manufacturers - door handles, grab irons, door latches, light fixtures, warning lights, are all off the shelf (once you find the shelf).
If you can find a unit that has the owner's manual, you're in the small percentage of buyers. All the manufacturers do things similarly, so a manual helps, but it certainly isn't crucial. A friend of mine is an ambulance electrician who worked on different units every day, from every manufacturer. He never even looked to see if there was a manual in the cab.
You mentioned Crestline - they have a solid reputation as a good ambulance builder. Some other brands out there have poorer reputations, usually for their wiring layouts and workmanship.