X2 on the Montana. But many other Garmin models will work fine - you just won't have that big beautiful screen.
Then, load Garmin Basecamp (a free download) onto your computer and a topo map of the area you want to ride. You can buy the Garmin 24k maps or download free ones from gpsfiledepot.com or other sources - search around on this site to see what others have been using. You no doubt already have a pretty good idea of the roads/trails you want to take if you have paper maps of the region. Using Basecamp, draw a track on the map on the computer following the roads/trails you want to ride and then load that track onto the GPS unit. I like to use a bright, contrasty magenta or red line for the track. Then as you ride, the map with your track and your bike on it will scroll with you as you move; as long as you stay on the track you can't go wrong. There are lots of helpful links on this site to the Garmin WIKIs that will explain how to install maps on your GPS and how to use Basecamp.