You can either find a used uhf repeater on E-Bay, or if you want a brand new one, I would suggest a Bridgecom Repeater.
If you only want to use one antenna then the repeater will require a duplexer or use two antennas that are separated by a couple of wavelengths.
Unlike amateur radio repeaters which are coordinated to eliminate interference, GMRS repeater pairs are not so you might want to do a little research on frequency pairs in your area. You will need to include either a pl-tone or digital coded squelch tones. Any user will need to have their radio programmed with these in order to access your repeater. Find a place to erect your antenna system and have fun. I currently use 20 feet of black water pipe with 2 yagi antennas which allows my antenna system some directivity. Vertex-7000 has everything you need including CW-Id'er.