Simrad makes the very best, but they are also on the higher end price wise, your looking about $2800+. A Garmin TR-1 is nice but does not have a display, and for the money and the so so reports I'd shy away from this. Raymarine offers some packages that start around $1600 and go up in price depending on features that will do what you want and are more affordable. All these brands should link to your current gps without a problem. There is a cheaper Raymarine unit, but at lower speeds it might not be well suited for you, it does not have a compass or gyro so it does not account for wind and tide well which for trolling purposes at those speeds you might want. If being a few ft off course is not an issue you can get this unit on ebay for around $1000, it is a basic unit and very simple to install and includes trolling patterns like clover, zig zag, straight line, and a few others. Your best bet is to go to a boat show and see the products first hand and see what suits your needs well and what is easiest for you to use. You may only need a very basis autopilot, or might perfer something that will be very accurate and top of the line like the simrad units.