I have a Garmin 541 GPS w/sonar. You can get it w/o the sonar sensor thus keeping the cost down and buy the sensor later. It does have a mounting bracket for the base and I bought the Winni map for $10.00 in a download from Garmin and it is great. When they get the new updated version (lake was surveyed this year) I will get that one.
We found some great pricing on this web site when we purchased ours a few years ago:
http://g.factoryoutletstore.com/cat/...-Sounders.html
They do have great customer service. Our 1st GPS arrived with a smashed screen and it was promtly replaced.