In the past couple of years, I have "invested" approx $120K into my C172RG for exactly this reason. The aircraft is a good fit for CPL training, SEA IR and for "hire and fly" and I can't think of a better option that prepares pilots for entering the GA industry as it stands today.
The aircraft is now SIDs compliant (and did require some corrosion work and fuel tank repair), undercarriage sorted with the better MLG actuators. Avionics-wise, it has an Aspen 1000 PFD, JPI EDM 930 and Garmin 750 / 430W as well as ADSB transponder. No vacuum system anymore - the SAI340 quattro saw that off! All integrated in setup by the avionics lads who did it as a complete package, so good enough for those wanting "glass" but in keeping with my preference for a traditional scan for trainees, I've kept a standby ASI, altimeter and VSI in the usual positions around the Aspen. Re-upholstered interior, and a nicely finished panel fascia and the aircraft should now be a good solution for several years to come, with weight savings to boot.
As above, you can get a 172RG quite cheaply - so spend the money on it and I reckon for about a total investment of up to $250K you will have a really good aircraft that fits your intent perfectly. The added bonus is that in "restoring" the aircraft, you get to identify and deal with the hidden gremlins and hopefully avoid them hassling you in the years ahead.