I've actually been using a Garmin Nuvi 2455 for several years. It's not a motorcycle specific design. The design has been superseded by newer models, so they are pretty inexpensive on Amazon (you can find a refurbished one for under $100.00). When I tour, I carry two of them, in case one breaks. I waterproofed them according to a tutorial I found on Youtube. One of the weak points in the Nuvi is where the USB power cable attaches; it's not a super-secure fit, so the connector tends to vibrate, and the vibration can weaken the connections. I solved this by semi-permanently attaching a short 6" cable to the GPS unit, and the power cable plugs into this short cable; that way, I never have to unplug the USB from the back of the Nuvi. The Nuvi is in a RAM cradle made specifically for that model, and attached to the accessory bar with a homemade RAM style mount. I also attached a homemade sunscreen, made from sheet plastic. It attaches to the RAM cradle with velcro.
I plan on my routes on a free program called Tyre to Travel. I have found it to be way easier to use than Basecamp. The downside to it is that you do need to have an internet connection in order to plan routes on Tyre, because it uses Google Maps. You can create standard .gpx format routes, and easily transfer them to the Garmin. These same .gpx files can be uploaded onto your phone and read by the OsmAND navigation app. It can come in handy to have the same route loaded onto both your GPS and your phone.
If you decide to go with the Garmin, make sure it has the Trip Planner feature, if you want to use the Tyre program to plan your routes.
The photo shows the Nuvi on the left side of the accessory bar. To the right is a homemade weatherproof box for my Droid Turbo. That thing attached on the center of the handlebars is another homemade cell phone mount.