You'll love the power on that unit, Rich. As for the transducer question, the dual frequency will only ever be useful if you'll be doing off-shore fishing. In fact, I just looked up what Lowrance has to say about that.
"Dual-frequency 50/200 kHz units are specifically designed for saltwater angling or tracking downriggers in the Great Lakes due to the capability of reading deeper and wider with the 50 kHz frequency. For superior inland and freshwater performance, single-frequency 200 kHz units are recommended due to the superior definition and target separation."
Good luck.