I think I may be able to help you out on both fronts.
I just purchased the navionics chip this year as well. I was having trouble with getting it recognized on my gps unit. I called Navionics customer support and they had me up and running in no time. I would recommend giving them a call as opposed to calling lowrance.
As for your prop question I have the same setup. My boat is a 1986 Lund Tyee 4.9 (thats 4.9 meters which is about 16.5 feet). I put a 50 HP Honda on the boat 2 years ago and I went through a bunch of trial and error getting the prop right. The difficulty was that the 50 HP is at the bottom end of what was really sufficient to power the boat. I found that the best prop was a 4-blade 10 pitch prop. Any pitch higher than that and I had issues getting on plane with more than 1 person in the boat. Find a marina that has some loaners and you can fine tune your prop until you find the right one that works on your boat. I have recently purchased a hydrofoil for the outboard that I am going to try as well. I am hoping that it will help me get on plane faster and easier. Top speed on my boat is about 22 MPH. One thing is make sure you are not over revving your boat at WOT.