Regarding the avionics upgrade options:
If your plane can do the required three different types of approaches and passes the IFR inspections, then stick with it until you finish your rating and decide much later on. If it cannot, then learn how to fly by instruments in your plane but rent a plane that can do the required approaches for a couple lessons including the cross-country and rent the same plane for the check ride. Flying the plane is basically the same no matter which type of approach you are doing, with the main difference being that a GPS approach gives you fewer things to worry about. (No need to listen and identify the VOR or localizer. No false glideslopes. Less jittery CDI. Etc.)
If you can fly your /A plane well and you can correctly operate the GPS in a /G plane, you can fly the /G plane just as well. Approaches are just a matter of following needles. The real instruments you are learning to fly with are round (six-pack plus engine instruments, CDI, and clock), not square (NAV/COMM, GPS, etc.)