The mileage is not an issue for the engine and probably not the Allison transmission either, assuming proper maintenance for both. Other running gear, e.g. wheel bearings, steering/suspension, universal joints, exhaust, etc. are likely showing signs of wear and tear or, e.g. the shocks, are flat worn out at 127k. The Bus will be on a Freightliner chassis, so I would plan on taking it to the Freightliner Factory Service Center in Gaffney, SC and having them go over the chassis with a fine tooth comb as well as changing all fluids and filters. That way you know you have a fresh start with a chassis in good shape. Figure on spending $2000 or so - just changing all the fluids and filters will run nearly $1500.
It is probably well into the second set of tires too, so new tires (major expense) will not be far off. Figure another $2400-$3000.