If you save the waypoints from the 520c they should load onto the HDS unit with no problem. They are in the same format. Make sure they are on the card.
Navionics chips can be better than the Insight mapping currently in the HDS units. Check out the body of water you are fishing and compare the data. If the added data on the Navionics chip is worth the money, pull the trigger. Lowrance is working on individual lake survey's and updating the map data that will be in the future HDS units.
The Elite transducer will only work in an Elite unit. The cable end connection is different from the HDS ducers.
Doc Samson has the best instructional DVD's available. Do a search on line for his stuff. Fish show up as little white specs on the Structure Scan. Because of the higher frequency you get a great pic of the structure and the broadband sonar shows fish the best hence the need for both types of sonar.