Anyone who has used several GPS units will have personal likes/dislikes that may be differnet from what you need. There are several different "motorcyle" GPS units (read expensive) but any GPS can be used. I have a Gamin Nuvi that is aobut 4 years old. It is not waterproof but I have used it uncovered in light rain (behind windshield on R3T) and in heavier rain with a clear plastic bag on it.
Try out the dispaly and controls on several GPS (take your gloves with you) and get the one with the features you want. You can get bluetooth for spoken directions, listening to MP3's, Xm radio, traffic alerts, etc. There are several companies that make a varity of mounts - Ram Mounts lots of options, cheap, bulk; Techmount pricey but look good; Powerlet which uses parts form Ram and Techmount but includes electrical outlet near the mount, if your unit has a none replacable internal battery this may be a good choice.