if you arent repairing what you already have and are purchasing new, purchase all parts together.
it may be that the current sender unit you have installed is bad, so buying a new head unit wont make it better.
i have a combo unit on my boat and awhile back the depth reading went wacky... i was told that i should replace the entire unit, but after doing some research i found that the transducer sales were fairly high from this particular manufacture, so i purchased a new transducer for $50 plus free shipping, and the problem was solved....
this doesnt mean you will have the same experience, but it does mean that you need to do some extensive research and make a reasonable purchase based on what you learn and what hope to achieve as the end result..
for some of us, its all about the money savings, and for others its all about the time and headache savings... and sometimes its just tough to decide which is the better route
