With no response from anyone else I think this indicates that no one else has personal experience with these either but I’ll try to help.
I believe the X17’s have grey connectors, (the network connection on the back of the unit could be blue), so if you had a 3000 and red network then you must have found a converter cable to connect to the red/black network. I don’t have any X17’s around to look at so I’m guessing.
If you have a converter cable to a red/black network and the LGC 3000 worked fine on the existing X17 then that’s the one to go with. I thought the X17 used to older 16W but that’s from my memory from years ago.
As for the transducer I think the connection on the back of the X17 is grey and those ducers are very hard to find. To get the model number you need look at the chrome tag on the ducer you have on your 18′ Starcraft and try to find one on some dealers shelf that he never sold or someone who has an extra one they never used and they still have it and will sell it.