That is an Eldorado National bus which has zero/nadda connection to Neoplan. They were originally built by the National bus company in CA until Eldorado bought them out about 20 years ago. They now are building the bus under the Eldorado National CA banner. They have been built in many different lengths and configurations. This particular model is now known as the ENC XHF.
Most were built on a Freightliner Custom Chassis XB chassis. The older vintage ones were built on Workhorse, Oshkosh, and Spartan chassis to name just a few of the OEM vendor supplied chassis that have been under that bus. The new ones are now being built on a chassis built by ENC.
The basic bus has been in production since the '80's. All of the glass except for the windshields are flat glass so finding replacement glass for those windows would not be hard or expensive. The windshields are probably still available from Eldorado National.
They are not a bad bus. They are not the typical plastic bus one usually associates with the Eldorado National brand. Even back in the day when National was making the bus they were made with a monocoque steel cage with metal panels. Some were/are made with aluminum but most were/are made out of steel.
No they are not built as stout as a school bus. This is not a bus you can raise the roof or add a slide. But if you can live with the way the bus is currently configured you should be able to make it into a nice conversion.
Probably the biggest gripe about them is most of them were put into airport parking lot or similar work so very few of them were ever ordered with big HP. Most did have highway gears so you could really beat feet between the parking lot and terminals. But since the distances were usually pretty short and terrain generally flat around an airport they really didn't need big HP.
One concern I would have would be with engine cooling. It is always a problem for RE buses. A bus that was designed and built for short highway hops and lots of stop and go work most probably does not have a cooling system adequate for going up a steep grade fully loaded on a hot summer's day.
XHF | ENC