I use a GPS extensively on an ATV and the group I ride with we all use GMRS radios with earbuds to keep in touch and to keep some foolishness going on while we ride.
This is a few years back, but the riders that had the Rino GPses never felt like their GPS/radio combination ever really excelled in both aspects as well as the radios and Gpses that those of us had that were a dedicated GMRS radio and a dedicated GPS. One thing to keep in mind is that if the Rino radio won't work (out of range) then the location of your buddie's Rino won't appear on your Rino's screen.
If the Montana is a little pricey for what you wish to pay, then I would suggest the Oregon 450 for around $250-$300. Some riders in the past year have started using the automobile Nuvis since you can get a lot bigger screen for around $100 that includes street maps and download the free custom maps and topos that you want. These Nuvis they are using are not waterproof, but seem to be doing ok. For the price of around $100 for a large screen, it is hard to complain much about it, if something did happen to it. I believe different model Nuvis, over the years have treated at what zoom levels custom maps will start to appear at differently and they don't handle creating tracks/breadcrumb trails nearly as well as a handhelds will.
If you are new to using a GPS there will be a learning curve in getting comfortable using it. Don't get discouraged and before long you won't go ATVing anywhere without it. It's great being able to zoom in/out and being able to see an entire trail system to mentally map out how where you want to go and where you are in the trail system at all times.