Here are pics of two I found for sale (do not have one of my own currently).
1998 dog-nose conventional Navistar with manual transmission.
1999 dog-nose conventional Navistar with auto transmission.
First, about noise. Your wife needs to understand that these buses are at their basics, commercial-duty truck chassis, and were not designed to be whisper quiet. Most with auto trans will have plenty of space around the engine area, and the engine is usually a good distance from the firewall. However, all is not lost here -- if noise is a concern, you can use DynaMat or some other sound deadening material (it quells heat as well, which is a plus).
Alternately, you can look at a gasser (quieter than a diesel) or a rear-engine pusher, they drive quite nice and are much quieter, although there are some trade-offs...
Front-engine: More noise, flat-nose are a bit hotter (again, DynaMat), compromised traction in mud and bad weather. Conventionals are easier to service, however.
Rear-engine: Quieter, cooler, better drive traction in mud / bad weather, steer tires more likely to skid / get off-track in mud / bad weather.