I have the Garmin GPSMap76. I have the lakemaster s/w loaded on it and love it. Lakemaster and Navatronics are both good choices. There are probably others as well, I just looked at these two and can't attest for the others. Garmin has their own navigation s/w as well. The choice of software will be easy compared to the GPS unit itself. For the software, look for what covers you area and what gives the detail you are looking for. Pretty simple.
When I bought mine, I already owned a garmin and had software and other accessories that I could use if I stayed with Garmin, so that narrowed down my search. I picked the GPSMap 76 because it had the largest screen of the handhelds, and it is a marine class GPS (waterproof, floats, etc). I looked at color units and chose the b&w because I didn't see that the color added value for the extra $150. I got mine on eBay for $175, and the lakemaster s/w for $50. You'll be hard pressed to do that well. I was very patient. I spent 3 months watching and tracking high bids so I knew what a good price was. And I got lucky.
Just make sure that the unit you're looking at has mapping capability. Not all of them do.
I agree with pirkaus. Take your time, do your research and you'll be happy.
