Here was my priority upgrades, accomplished gradually over 10 years or so. Based on flying IFR:
1. WAAS GPS (GNS-430W)
2. Modern audio panel (Garmin GMA-340)
3. Autopilot (STEC-20)
4. ADS-B out and in (Lynx NGT-9000)
5. EFIS (Dual G5s, removal of vacuum system)
6. Upgraded #2 NAV/COM (future, my current #2 still works and is rarely used)
I'm at the point that I will soon have to go back to #1 and go through the list again. I could add engine monitoring (#7) but I managed to do just fine on the first engine without.
Along the way, I also redid the interior upholstery and plastic. Some of that I did myself. THat was not a critical safety of flight issue, but was a welcome upgrade. I also ran out my engine and overhauled it while doing an engine and prop upgrade. The new engine and prop is sooooo much better than the old ones. It's nice having items you can personally take care of properly from the start.
Pretty much, upgrades and replacement happen continuously.