2. I have heard about people having to convert from seatalk to NMEA -
but the S3 has NMEA in/out connectors so I assume I dont have to do
any conversion?

Yes, you can use the S3's NMEA input instead of a separate
NMEA-Seatalk interface.

Thanks Peter,

Spoke to a guy today who confirmed all of what you said, bar one
thing, he stated that the NMEA output from a Raymarine S3 corepack
only has sentences directly related to the autopilot (ie no wind,
depth, sea temp).

He said to get all the sentences you need to connect to Seatalk, and
therefore need a Seatalk-NMEA converter.

Thanks

-Al