You might want to verify that the transfer switches you purchased are compatible with the generator you purchase and if two transfer switches can be used with the generator that you purchase. Generac offers transfer switches for their air cooled generators and different transfer switches for their water cooled generators. Some older model transfer switches are not compatible with newer model generators.
Kohler just introduced new water cooled propane powered generators. The new models are 38 KW and 48 KW, around $14,000.00 and $16,000.00 MSRP. The difference in price between a 38 KW and 48 KW generator is about $2,000.00. Here is a list of generators including 38 and 48 KW generators. You will find that a water cooled generator from 38 to 48 KW cost about $10,000.00 over the cost of a 20 KW air cooled generator. It is important that you select a qualified installer because even the most reliable generators will not be satisfactory if not installed properly. Here is a price list for Kohler water cooled generators:
General Electric/Briggs and Stratton have just introduced new model generators, you might want to check them out as well. Home depot lists Kohler, Generac and General Electric water cooled and air cooled generators. I strongly recommend downloading and reading the owner manual for each generator that you are considering, to learn the maintenance intervals advantages and limitations of each generator.