There are tons of different sizes of E350 based busses. Mine is about 20 feet long all together, with an interior floor space of about 11x7.5 feet. This may not be of much importance as mine is a school bus body and not a shuttle, but width tends to to be a consistent 7.5' in a lot of these busses. Length can vary widely...I think I read through some of sdwarf's build and remember seeing that his was a bit longer than mine.
As far as miles go, the 7.3 diesel engine in these things will last 300,000+ assuming it's been well maintained. One thing in specific to ask the maintenance guy, if you have access to him, is about the coolant additives ( I think they're called SCA additives). Running this additive is crucial, as the engine will basically eat itself without it (that may be a little melodramatic, but the additive really is important) . You can buy a coolant test kit to check if there are proper amounts of additive in the system, and the additive itself is only like $5 per bottle from the Ford dealership or Napa, I just picked some up a day or two ago. And of course routine things like regular oil changes etc.. My bus currently has 228?,000 miles on it, I think I bought it at 223,000 or so. It's dragged me through some of the most demanding environments I can think of (steep, loooong grades in the middle of the SWELTERING hot Mojave, etc.) and for a few months I even daily drove it through heavy city traffic
A lot of these 350's have the E4OD trans though, which unlike 7.3 diesels, is not known for being especially reliable (IIRC, it doesn't pump fluid when in reverse..wtf?) Watch out for a slipping/hard shifting/code-throwing tranny (its throwing a code if the "Overdrive Off" light on the shifter is blinking).