Here's a flat recommendation. Get you a 520C with an external antenae and you're done. Might could find one with an internal if that floats your boat. I can't yet recommend the HDS units because I haven't seen them, but in a 5" color with a very good GPS and excellent traditional 2D sonar, the 520C can't be beat....plus you're liable to find them on closeout somewhere since they've been discontinued and have enough $ left over to stock up on some jig poles.
At $600 you're out of the market for SI. To me, the Lowrance traditional 2D sonar is a better and easier picture to decipher. I agree on the Garmin being the best GPS but the 2D, to me, is subpar at least until you get in to the very expensive units.
Anyway, that's my story. I'm sure someone will disagree, but they'll be wrong as I am extremely sure this is what I recommend.
Wannabe...