Hey J,
Depending upon your desire to expand your Lowrance system, you could go with an HDS 7, add Structure Scan and you'll be in business.
The advantage to going with an HDS from the start is expandability.
By simply adding system modules you can end up with a stereo, radar or even an autopilot.
I realize that not everyone or every boat requires or even needs those features but if you ever succumb to the dreaded "two-footitis" it's nice to know that your existing gear can easily handle more features and you will not have to learn how to operate a completely new system!