Not only will a decent GPS unit allow coordinate entry, it'll allow you to choose which coordinate system you want to use.
While it's true that virtually all GPS's will allow that, the distinction, I think, would be in the types of maps that you can load. The Garmin handhelds that are made specifically for off-road use would have detailed terrain maps with contour lines and waterways... but the downside is these usually have quite small screens.
Not sure why you'd want to connect directly to your battery... but if you want a clean installation with the minimum amount of wiring showing, I'd run the wiring down through the handlebar bars ("inside" of them). Then, from the front inside of the fairing, run the wiring into the left or right cubby area in the fairing. From here, you'll either have to remove some side panels or fish the wiring through to reach the battery area.
Instead of doing all that, why not just splice into the 12V power socket?