Nice body. Lot of potential. Make sure you have your insurance lined up before you get involved with a bus conversion. It appears to be very problematical in some states, not too hard to do in others.
CNG tanks operate at VERY high pressure, 3,000 psi or more. They have expiration dates on them, a sticker near the fill saying something like "Do Not Use After xx/xx/xxxx."
Legally, the refueling place is not allowed to refuel it after that date, or if the sticker is missing. They are also supposed to refuse to refuel it if there is any visible sign of damage to the tank, a lot of corrosion caused by road salt, etc. A new CNG tank will set you back several thousand dollars. Most CNG vehicles are scrapped when the tank expires cause it's usually not worth it to put a new tank in.
I think the tanks are good for ten years, maybe twelve. So 2011 vehicles might have 5 years or a little more left on them.
My personal opinion is that CNG vehicles should be left to the fleet operators, taxicab companies, bus companies, municipalities, etc. They usually have their own refuel facility, and they have their own mechanics who are certified for working on CNG systems. For an individual, the headaches WAY outweigh any possible advantages.
If you REALLY like these busses, it might be possible to have them converted back to gasoline operation. You'd have to investigate the cost and practicality of that option before getting involved in the project.
