The Kenwood 805 is a good choice, and you should be able to find them pretty cheap.
FRS/GMRS gets pretty confusing since there is some overlap. There are 7 GMRS frequencies (limited to 5 watts) that are shared with FRS. If you were properly licensed on GMRS, it would technically be legal to use those 7 shared channels to talk to your buddies on their FRS radios.
GMRS, of course, allows you to use up to 50 watts.
Word of caution though, you really don't want an antenna running 50 watts that close to you, as it would be on a motorcycle. The traditional "motorcycle" radios used by the police are often limited to around 15watts.
Both Motorola and Vertex make dedicated motorcycle radios. Actually, they are mobile radios with a specific remote head designed for weather resistance and for mounting on a motorcycle. The radio "guts" are mountable in a box. They will be harder to find and likely more expensive.
A standard mobile radio will work, but you will need to protect it from the weather. You will either want a headset, or a good external speaker that will allow you to hear the radio.
I ran GMRS on my ATV's for a few years and then switched to amateur/VHF. It's nice having the extra power, but I always needed a headset or a big speaker to be able to hear the others over the noise.
Another thing to consider if finding a place to mount the antenna. If you have boxes to mount it on, that works, just make sure you have a suitable ground plane under the antenna, or you are using a ground independent antenna.
The other option is to use a hand held, and power it off the motorcycle battery, install a permanent antenna, and hook it up to your intercom system. There are a lot of systems out there designed for this.
Interfacing a mobile radio to a headset can be a bit more difficult. But, searching the web and being ready to experiment will lead to success.