first you will have to decide if you want to go with enamels or acrylics. In the past i did my MERDC finishes using Model Master enamels, as especially their FS30118 looked spot on to me. Lately I have been switching to acrylics only, but haven't done any MERDC / Dual Tex yet. There is a couple of different brands providing the FS colors you are looking for - e.g. Model Master Acrylics and Lifecolor (in my opinion, the later are a bit tough to control with the airbrush). Tamiya, my favourite to squeeze through the airbrush, doesn't provide the range of colors you need. You could use XF65 Field Gray for FS34079, but Field Drab and Sand would have to be mixed.
Hope that helps,