If going to a shoot thru the the AIRMAR P79 (Same as the P66) is the entry level unit but is not easily configured in a Lowrance plug .
The next level is the M260-Bl and is compatible with the LCX and HDS series.
(But, Lowrance units are not the best choices for "shoot thru " the hull transducers.)
If you choose to go thru the hull or transom you have wide choice depending on your fishing conditions and depths.
How deep and what do you fish for ?
The GPs antennas for lowrance:
LGC-12W (Lowrance part number 112-06)
Fits many units including LCX- 15mt,15ci,15ct,16ci,18c,19c,104c.
NMEA 2000 GPS WAAS antenna with 5 htz update map rate.
Fits all HDS units.
This might also work:
Fits all NMEA 0183 and NMEA 2000 instruments to provide Lat, long, COG, SOG, time and date, magnetic variation and satellite status.AIRMAR might be able to provide a Lowrance cable option.