Why not just install the transducer that comes with the chartplotter? It couldn't be easier- just run the cable to the proper location in the hull and fix it in place with silicone. This way you'll have another (redundant) depth gauge. Also, I don't think you can mix and match transducers. Each depth gauge comes with its own transducer and a specific length of wire between the unit and the transducer. A shorter or longer length of wire will affect the readings.
Neither the 521 nor the 527 come with any kind of transducer. The best transducer to use with it would be the Garmin/Airmar DST800, which connects via NMEA 2000, and gives depth, speed, and water temperature. I have one myself.
This does require a through-hull fitting, yes. If you can live with just depth, you could probably pot a transducer (no through-hull).
The Lowrance Elite 5 does not have NMEA 2000 which would be tremendously limiting. Personally, I wouldn't buy a plotter without it.
To the OP, you could start with the Garmin plotter and transducer combo, and then you will have an NMEA 2000 network. The expandability is excellent. Extra displays, transducers, autopilot can all be easily added and will share data.