sailboat upgrade path

[snokid]

Chuck

I have the boat hauled out for winter, so it's time to finish the install, so far everything has worked no problems

Axiom 9' without the sonar option

I bought but now am wondering if I bought wrong, this is the transducer I bought
Raymarine Model No. M78717
Mount: Thru-hull
Housing Material: Bronze
Connector: Spade connector for Raymarine instruments
Cable: 45 ft.
Frequency: 200 kHz
Cone: 13°
Power: 250 Watts RMS
Max. Depth Range: 400 - 600 ft.
Acoustic Window Material: Urethane
Compatible Hull Materials: Fiberglass or wood
[censored] Diameter for Installation: 2"
Dimensions: 4.87" H x 2.94" OD

If it's the right one I do have another question.
I read somewhere bronze isn't the best for salt water and I should use plastic, can I convert this one to plastic?

Next is speed temp, getting confused which one to get I think I have it figured out then I read is not good below 2 knots
So would you direct me to which one to get? I need 2" thru hull forward of the keel.

thanks for all the help
Bob
I have the axiom 9 mdf gps, itc5, i70s, quantum q24w, st60 windvane transducer, s100 remote, and ev200 autopilot