What I have learned in my search - the items that impact Vref value the most are Logbooks, Damage history, Total Time, TSMOH and avionics. The avionics give zero value. Add $11 for every hour under .5*TBO. So in this case you would add about $5500 to the base Vref value. Paint and interior rated at 7 is zero addition or deduction - add or subtract as usual. Avionics credit is about .25-.5 the installed cost of the units depending on how new they are. I think the current addition is $5000 to base value for a Garmin 530W. It would cost at least $10000 to get one used and installed.
Base price for a 76F according to Vref is surprisingly $53,000. Adding the low time and assuming 7/7 int and exterior it is $58.5 according to the formula the AOPA lenders will use.
Now the question is WHEN were those 500 hours put on? If the overhaul was 15-20 years ago...its very risky. If it flew 500 hours in the last 5 years...giddy up!
An F is a good airplane but it will always be an F and perform about as well as the earlier Fs that may have a base price of 40K. It will never be a J. Base price of a 77 J is $71K.
Now the question is - what is the aircraft worth to you? The airplane that I am in the process of buying only had a Vref of $38K but I'm going to pay more for it because I know the history of the plane, owner, type of flying that was done on it etc. For an airplane, its a very low risk purchase and it meets my mission requirements (IFR in congested airspace). I have found much better deals but they all came with airplanes that had some "neglect" in their history.
I would say this - if this airplane has a solid maintenance history, all flying on this motor in the last 5-10 years, responsible owner and meets YOUR current requirements and YOU are willing to invest in avionics then its probably safe that you will not end up "under water" as far as the bank is concerned. BUT - if you could find the same airplane with the avionics already installed it would be equal to about 1/4 the price of getting it done after the fact.
Vref/BlueBook does not do justice to the current price of installed avionics. It will be much cheaper in the long run to find the airplane you want with the avionics installed.
STEC 30 is only a 4-5K valuation increase but will cost you probably $20K to get installed in todays market.
Good luck - finding the right plane is hard!
Further example - Using Vance's airplane: 55K-11K (motor)+$5000 (GPS) = $48K but does not account for inter/ext condition, total time or other avionics/mods.
Edited to add: Vref is a "retail value" price or what a dealer could get away with listing an aircraft for...not necessarily a selling price.