The way to go right from the start with nothing to begin with is to get a new GPS capable depth finder with a Lake Masters chip. Like You I fish N/W Ontario near Dryden. Eagle Lake has a LOT of reefs and shallow rocks.
The GPS is more useful at times for avoiding those and/or mapping routes through reefy sections of lake. Obviously, fishing a lake that is bigger than 100 square miles brings other considerations like finding your way from point A to point B or more importantly, from out on the lake back in to the landing when there is nothing but trees along the shore. Fog and rain can make things very difficult and dangerous. Because of that, I would advise a mapping GPS in the depth finder and a basic hand held backup like a Lowrance Ifinder-GO which costs next to nothing and give 48 hour battery life out of a SIRF-III chip set.
Another reason for the depth finder/GPS advice is to get a new chip set and be able to take full advantage of the new satellites and improved accuracy. I bought a Humminbird Helix 7recently and some of the LakeMasters maps for newly surveyed lakes are hyper accurate and can show very small structure that can be very productive. That can lead to a very good place to fish with a much shorter boat ride to get there which means a lot on big lakes. It takes both the GPS and the sonar to go quickly and accurately to a boat size piece of structure 25 feet down in a big lake. The contour lines on most of the lakes I have used the new units on have been very accurate. Not always "right", but accurate. The lake levels can and do change which makes them wrong. But if a contour line was at 15 feet when the survey was done and the lake came up 2 feet the 17 foot depth will still be accurate. That is important to keep in mind with big reservoirs like Wabigoon or Eagle. Here in Minnesota, I find that it is pretty common for nice fish (bass) to orient to a very specific weed and that being able to mark a waypoint a little distance off it and then cast to THAT weed.
A down side of the new depth finders is that they can be pretty complex to get tuned well. A couple weeks ago I spent the better part of a day fishing tuning a Humminbird Helix 7 on Eagle and got it to the point that I could tell with pretty predictable accuracy the size of walleyes in 20-25 feet of water. My home unit is one with Down Image sonar as well and that capability is well worth buying a new unit for alone. Down image is easy enough to tune for just telling you what you have underneath you, but similarly tedious to tune perfectly. It seems to me that the tuning is also variable by water temperature and organic content
I like the Helix units from Humminbird, but I wouldn't begin to say they are better/easier/more accurate/useful than any other brand. I don't know and really don't care. I do know that I have more access to them and am more familiar with them.