red-dot is good.. transair makes an evaporator deigned specifically for the IC RE bus to be mounted upfront and fit in.. many RE busses already have dash vents.. or swivelling defroster vents you can spin around in summer to blow the cold air on you instead of the windshield.. you can likely adapt it..
years ago I fashioned my own Dash air into my defrosters.. that was completely custom..
condenser wise.. a sma skirt mounted condenser may be best so you dont interfere with the engine cooling which can already be fragile in an RE bus..
there are different companies making Compressor brackets.. Bracketry systems is big into bus A/C brackets for many engines.. 'CPI' Component Parts | World's Premier Designer & Manufacturer of Vehicle Adapter Components
makes compressor brackets too..
transair manufacturing, Thermoking, red-dot, AP-air are all comoanies making bus air conditioning gear..
vintageair (mainly old car retrofits) is more than willing to work with you on custom projects.. I have bought parts from them for some of my completrly custom A/C installs ive done over the years (and ive done a lot)...