I don't know much about the technical issues you're asking., but my question to you is this..... do you need AIS and radar? Do you go out in the dark much? Or in the fog? If not, then AIS transmit and receive will be good enough alone.
The reason I say this is because I have both, and apart from one trip out in dense fog, I've never used the radar. Ok, so it is nice to know it's there but is it worth the cost? You could always add it later if you feel you really need it, but for now I'd just go AIS.