For the last nine years I've been the curator of an airborne museum of ancient avionics, that looks a lot like a C-172N. The show is headlined by the 40-year-old original KX175B navcoms and KR85 ADF, all controlled by the quaint, but TSO'd, KMA20 audio panel. More recent additions were a KN64 DME and a KLN90B (VFR) GPS, but they're no spring chickens, either. The 21st Century is represented by the Stratus ADS-B out transponder.
So really, you just have a radio problem. ADS-B is solved already.
I no longer fly IFR. I live in a state where clouds are discouraged, and those few that do appear are often filled with lightning, hail, and other nastiness. Primary VFR navigation is handled by the KLN90B and iPhone/iPad/Foreflight, though sometimes I'll fiddle with the VORs for old times' sake.
Do you even need two com radios? There are some com radios that can act as an audio panel/intercom as well. OTOH, making it well equipped for IFR may improve resale value somewhat if the next guy wants to fly IFR.
I plan on keeping the airplane as long as it will have me.
So that makes spending a little more money worthwhile.
Where's Nancy Narco when you need her?
Ugh. Narco. May have been great back in the day, but now they're dead and gone, and good riddance I say. Dealing with them before that was annoying. Nothing like a hole in the panel for 14 months for a simple radio fix.
Sadly, King is following the same path Narco did.
Well... Depends on the budget. On the plus side, you're planning on keeping it. On the minus side, well, that 172 ain't ever gonna get any faster no matter how many dollars you stuff in the panel.
High end: GTN650, GMAsomething, and GNC255. Much nicer GPS, modern audio panel, and a very nice #2 nav/com.
Medium: Pair of digital flip-flop radios - You might be able to find some good deals on the used market. I have a KX155 that could use a little work that we just pulled out of the Mooney (in favor of a PoA-approved GTN 750) and I'd be happy to put you in touch with my avionics guy to see exactly what it needs. (They've got a great component-level tech there - 30+ years doing component-level repairs on avionics, and has saved my bacon from much more expensive unit swaps a couple times.)
Cheap: If you can find someone to ring out the wires and check the connectors and test the radios and isolate it to surely be a radio problem, aside from the TKM MX series, you might also consider a MAC 1700 as a replacement. We've had three of them in club planes over the years and been pretty happy with them. They combine the top-notch radio guts of the Kings with a replacement for the Kings' major weakness: The mechanical tuning. It's basically a King radio with a digital faceplate on it, with flip-flop as well as memory storage for up to 10 frequencies on each side.
But really, unless you spend your money on newer gear that gives you some new connectors and wiring anyway, you're going to spend money on a person spending a bunch of time troubleshooting the problem. While you only get back maybe half what you spend on avionics when you do sell, you'll get NONE of the troubleshooting money back. If you think you'll have it for a long time and be flying for a long time and you have the coin to do it - Shoot for the moon and get some nice gadgets that you WANT instead of just what you NEED. If you want to keep it low budget, wire up some sort of digital flip-flops and you're good.
