From a legal perspective, you must get a license from the FCC to have a commercial repeater. If the current FD license is only for individual radios, you will need to submit a new application and obtain a coordinated frequency from an FCC approved frequency coordinator. Unlike a ham radio repeater, you cannot cobble together any old transmitter and receiver to build the repeater. It must be FCC certified/approved for Part 90 service - more specifically, the transmitter must meet this requirement. You will need to supply antenna height information, antenna gain, power, expected radius of coverage, your assigned coordinated frequencies, etc. as part of the application.
From a technical perspective, do not guess at coverage. "Flat" land is quite relative. You should use appropriate software to model your intended repeater location, height, power, receive sensitivity, duplexer loss, and antenna system gain along with the local topography to ensure that you obtain the intended coverage.
A free, on-line program that you can use for coverage modeling can be found at:
http://www.ve2dbe.com/rmonline.html. There are several other packages that will serve the same purpose.
- Glenn W9IQ