I decided in 2017 to bite the bullet and buy a GTN 650. Until then, I was fine flying in the system using VOR, NDB, and DME, but ground-based navaid decommissionings were accelerating, Nav Canada was deactivating most of the Victor and Romeo airways in my part of Canada (the
Quebec City-Windsor corridor), and both Canadian and American ATC were increasingly vectoring me enroute instead of giving me own-navigation routings, so I saw the writing on the wall.
Fortunately, there are inexpensive options for IFR RNAV support now, much more so than there were for me in 2017. If you need just a GPS (keep your old radios and transponder), Garmin's
GPS 175 is only $4,995 MSRP + installation, and probably less from some dealers, which is far less than an ancient used GNS 430W went for on eBay just a couple of years ago. If you want to replace the transponder as well, you can get the
GNX 375 combined WAAS GPS and ADS-B in/out transponder for about $7,500, which isn't that much more than I paid just for my GTX 345 transponder in 2017. Just make sure your potential partners are not closed to the idea of upgrading in the future, if you want to use the plane for more than only IFR training.