I know there are several threads along these lines, but I'm still unsure of which bag to purchase. I'm looking for a bag to hold and protect the gear while transporting via car and for short walks. I need something that I can sit on the ground, that provides easy access to the gear. This is the kit:

Mamiya RZ67 with 180mm and prism finder attached.
Space for one or two additional lenses: 65mm and 110mm.
2 to 4 120 backs
Space for film, cable releases, filters, etc.

Any suggestions appreciated. I am going to take a trip to our local camera shop to see if they have something, but there inventory is limited.