I bought my Gast pump from an online Septic pump store first and Ted helped me build the rest of the system. Vertex makes the Cadillac of diffusers. I was lucky the pump I purchased ahead of time was sufficient to run the system that I wanted.
I wanted a rotary vane pump for less maintenance, good durability and they are relatively quiet. This is the one I have:
http://www.gastmfg.com/support_documents/STD463rA.pdf and it runs my XL5 nicely at 13' deep, I don't remember the length of run.
http://www.gastmfg.com/product_overview_compressors_pumps.aspx 
