I investigated using a 35ft Postal Crown.
Postals have a lot going for them. Driveline is compatible with Crown skoolies so parts supplies are VERY good, better than something like a GMC bus of the 50s/60s/70s.
The frame on a Crown tandem 40ft skoolie is deliberately weakened behind the rear axles as a "crumple zone safety feature" - as a result, turning the back wall into a ramp door and adding a rear motorcycle/ATV garage would take some serious engineering. This is NOT the case with Postals, the primary frame rails run right to the back bumper if this is a REAL Crown factory postal. ("Fakes" made by people out of skoolies are out there too and should be avoided unless you're experienced enough to confirm that their engineering was right.)
The downsides as I understand them:
1) Engine access sucks. At a minimum, it's annoying to deal with under there.
2) My possibly limited understanding is that the mid-engine Crowns with Detroit 671/671T two-strokes use the same engine block as the normal upright versions. Only the oilpan and oil pickup change to convert an upright block to layover. This is NOT the case with Cummins Big Cam or Small Cam motors - they have to be built as laydown blocks and may be hard to come by if you have to do a total re-power. As a result, I would pay more attention to the motor's condition than usual for the price range involved...
3) As with ANY old bus, a professional inspection for rust is critical. Crowns aren't bad in this area but many Postals had a lot of steel in them and if you don't know where to look you can get way screwed.