Read the Garmin literature pretty closely. The built-in charts may or may not cover the Canadian side of the Great Lakes. I had to buy another unlock code to put the Northern Channel of Huron on my gpsmap 76cs, but it was worth it, sailing up there was exquisitely beautiful!
Last year's crop of Garmin 400/500 series chartplotters will be discounted now or soon because they have introduced newer versions with faster processors and N2K connections; probably not something you would need.
A caution about adding another depth transducer over your ST-40. There will be contention between the two because they emit at the same frequencies. It is likely you will have a problem with one or both.
Raymarine does have a little black box that will let your seatalk
instruments talk to your garmin. With that you can display water
depth, speed and possibly wind
data on a Garmin Chartplotter. Its called a Seatalk/NMEA/PC interface (or something like that) and its reasonably priced.
Go to a store and look at the Garmin 400 series and 500 series together. Remember that you will be seated or standing fairly close to the helm when you are using it, so see if you can get by with the smaller screen
. It is cheaper and uses less power, but it has all the functionality of the 500. Learn what the differences are between the different units; it has to do with the resolution of the screen and which charts are preloaded. You can add Canadian charts from the $160 Bluechart CD and load them into either of these chartplotters wih common SD memory cards.
What ever you buy, make sure you connect it to your VHF radio
and the Coast Guard's Rescue
. This is a no-brainer. No DSC
button on your old VHF
? Buy a new radio
too. BEFORE you spend another dime on the boat! Yes dear, before you buy new curtains....
[drum roll] AND, if you have an autopilot
, both of these will drive it! SLICK!!!