I am not a bus expert yet, but there is something on the Thomas I saw that concerned me.
The rust on the bumper isn't a big deal, it doesn't look to be too deep, however, with it being on the bumper like that, AND it is a Mass bus, Id be cautious about buying it. Ask for pics of the undercarriage, especially around wheel wells, and fittings for any of the lines. I learned the hard way on mine and I am in the process of rebuilding the floor structure above my rear axle.
And everyone needs to quit hating on cat motors. They aren't THAT much more to fix, and they seem to be in everything from bulldozers to yachts! Plus there are a ton of dealers on the interstates if you need a repair.
The 3126 versus the 3126b is efficiency. The mechanical motors are definitely easier to work on, but the electronic ones get better mileage. I think porkchopsandwiches and wmkbailey can answer exact mileage as they have had theirs out on longer road trips.
Whatever you get, just be ready for a bit of work, a lot of advice, and even more fun when you start and finish your conversion!