If you go with the 740s then you have the option of adding radar later if you ever choose.
As to the transom mount transducer for that depth...500' and less.
A 4 element 1 kw TM258 is a good option...and the Tm260 is even better.
Using the P66 at 50 kHz as the benchmark:
The Tm258 is 25 times more sensitive..and the Tm260 is 50 times better than the P66.
Make sure whatever you get is a mix and match connector system..that way you can your the same transducer with almost any other sonar manufacture.
Mix and match.