I think we've got to get used to the fact that our avionics are going to cost more than a new factory engine, possibly be most of the value of the aircraft.
There are a bewildering variety of installations available, have a look inside some of the new permit machines.
Personally, if your flying is strictly VFR, then I would pay for a refurbished vacuum AI and DI, for those times when you're going to inadvertently enter cloud, or intentionally for a short period. If you are going to do any serious IMC flying, then I'd advocate an autopilot. I've flown with the new generation of Garmin and they are lovely, a major step forward from the old Piper and Bonanza models. So, if you're going to do that, then you'll need some form of GPS to drive it, Avidyne is nice, as well as the Garmin GTN750, which I've used in a Rockwell Commander. I thought the touch screen would be a pain in turbulence, but it's easy to rest your hand on the moulding on the side to steady yourself.
You mention GPS approaches, so I'm guessing you are IR(R) rated. At the moment, they are few and far between in the UK, a national scandal which the CAA still haven't addressed. At present, it wil be mandatory for IR holders to be PBN-rated by next year, though I don't think there are plans to extend that to IR(R) holders.
There's plenty on YouTube about various people's installations but none of it is cheap. If you think of a number, probably add 50% to it by the time you're done. i.e. £20,000 will likely actually be £30,000. I'd say the latter is realistic to re-equip a 172. Then, you're going to look at the 25 year old paint and interior and spend another £25,000. Go for it, it's only money and you're only here once.
TOO