Yeah, for a 2000 model even with 4WD, I'd try to get him down to under $10,000.00. But, considering it does have 4WD and even if you do have to pay the full $11,000.00, you won't be getting ripped off.
I don't have any experience with Demers, so I can't help you there.
Having any information on the electrical system is definitely a plus. If you get it, look for a set of yellow certification stickers either on the left front compartment door or inside in one of the cabinets. On that sticker there should be a WO# (work order number) or at least the VIN number. Call the manufacturer, give them those numbers, and they may be able to email you PDF copies of the work order itself, which will contain all kinds of useful information.
It sounds like there may be a small air leak in the system causing the compressor to keep running. That's not unusual in a vehicle that old.