The S-250 is very popular, and they can be hard to track down. You'll see them on E Bay or Graig's List sometimes, but they are where you find them. Check with any ham radio operators you know who are into field trials, and they might know of an S-250 that is available. The weight should be about 750 Lbs empty. They used several different types of airconditioner on them - including right rear mounted, front mounted, and recessed into the passenger side. There were a bunch of ACs released about 10 years ago that are still floating around new in the crate - I bought a 6000 BTU AC last year, and found that the AC was in 100 Lbs of lumber and packaging. The AC was MIL packaged, and once I got it out of the box and the barrier bag removed, all I had left to do was spend a few hours scrubbing tape residue off the sheet metal. It needed a freon recharge since the ACs service valves were an old design that was never leakproof - I updated the AC to Sporlan valves, and it works fine. Its about the most crowded 6000 BTU AC I've ever seen - plan on tunneling if you have to do anything to it. It was a MIL-STD designt hat was built by multiple MIL contractors. Some parts are standard, and others are unique to each manufacturer.
What are they worth? Good question - it depends a lot on condition. I've walked away from some that stank of mold as soon as the door was opened, and I bought one very reasonably that had a fork lift hole in the side. I used the procedures in the TM to patch the hole - thankfully, they had used spray foam to seal the hole before it became waterlogged. Once I leveled out the foam, I used some epoxy and countersunk pop rivets to fasten a patch to the skin and to bond it to the foam.
They seem to run about $ 1000 +/-. The big GOTTCHA is where is it compared to where you are. A cheap shelter that's 1000 miles away might cost a lot more than one with a higher price that's 50 miles away.
They fit a 105 trailer just fine - trhere was a thread a while ago that showed cutting an M101 trailer and moving out the wheel wells a few inches so an S-250 would fit.
My favorite part of the shelter is definetely the door. hands down best door on any camper ever. 
