I would recommend a motorcycle-specific GPS. They are built to be operated with gloves and are weather sealed as well as being vibration and shock resistant and designed with handlebar mounting in mind. Because of the more durable design and weather resistant design, however, they are significantly more expensive than the average in-car GPS. Garmin's Zumo series of motorcycle GPS receivers, for example, run $800 to $900.
As an alternative, many hand-held "hiking" type GPS units are also waterproof and shock resistant and will have an optional handlebar mount (usually with mountain bikers in mind, but they will also fit motorcycles). These cannot be hard-wired into a motorcycle's electrical system, but with NiMH batteries they typically last 8-12 hours which is sufficient for a day of riding. The map coverage (mainly in terms of attractions and points of interest) isn't as good as dedicated on-road GPS units, but for basic navigation purposes they work just fine. I have a little DeLorme Earthmate PN40 that is fantastic and works just fine on back roads here in PA (as long as I have the right region loaded into it). Unless you're looking to go riding on ranch roads without pavement, something like the PN40 would be just fine.
As far as updating maps, usually you don't need to. Major interchanges don't change that often and even when something does change, as long as you have a half-decent sense of direction you're fine. I bought an automotive GPS (a Lowrance) about 4 years ago and I've never updated the maps and it still gets me around just fine from Maine to Washington (state). Updating the maps is nice to be able to do every now and then, but it's not something you have to do every year or even every 5 years. Rural roads change even less; most of the updates to maps are the addition of new subdivisions, not taking whole roads off.
ZV