Exactly what model Lowrance unit have you got? Some have more capabilities than others. So that’s question 1.
Question 2 is - 3 part question - what depths do you mostly fish 90% of the time, what species do you target, and what is the max depth and how often do you go that depth?
With a thru hull transducer, most likely you will be looking at something from Airmar, or a Navico branded Airmar. You are going to have to cut a hole thru the hull anywhere up to around 60 mm diameter, so you need to figure out where is a good spot somewhere within about 400mm of the transom end of the hull. And on the inside you will need another 25mm diameter to fit the nut to hold it in place and to have some clearance. More powerful transducers will probably need bigger holes.
Those thru hull transducers typically come in 3 versions suited to the amount of V in the hull where they are going to be placed. The 0 degree model is for boats with a keel flat. The 12 degree is for hulls with 8-15 degrees of V, and the 20 degree is for 16-24 degrees of V.
Link below is to the entry level traditional (non-CHIRP) transducer - a B60/SS60 - its equivalent to a P66 transom transducer. Comes in a SS housing to suit alloy hulls.
Have a read of the brochure as it gives you an idea of install requirements and the capabilities of this particular model. First job is probably to figure out if you have a suitable spot to cut the hole and mount it.
http://www.airmar.com/uploads/brochures/ss60.pdf