That is *WAY* overpriced. The info is wrong, and incomplete. It is *Either* a Thomas, or a Blue Bird, but not both. You'll want to find out which engine it has, don't simply accept a generic "diesel" as an answer, you want to know which manufacturer and model. Examples include IH T444E or DT466, Cummins 5.9 or Cummins 8.3, Cat 3126 or Cat C7. There are a few others but these are what you're likely to find in many buses. Find out what transmission is has, and again, do *Not* accept a generic "Allison Automatic" (pretty much the only company to supply automatic transmissions for school buses). You'll want to avoid the AT545 (and its AT-variant brothers), it's a good transmission for a lot of stop and go use but ill suited for a highway cruiser. The MT645 is somewhat better but still may give a limited top speed (no overdrive). Lastly see if you can get the rear axle gear ratio. This, along with your transmission type, will determine your highway cruising speed (a low geared axle with an overdrive will go faster than one without overdrive). Typical ratios might be 5.38 (with overdrive is alright, with no overdrive you'll top out around 55 MPH), 4.44 (with no overdrive should top out around 65).
Keep in mind a larger engine will produce considerably more power for highway cruising, and the fuel economy difference will be negligible.