The refrigerator section does not cool well enough and food is rotting. Unit is around twenty years old minimum. Should I buy anew one or replace parts which was estimated at $4500?

check to be sure condenser fan motor is still working bottom back of refrigerator , also is there frost at back of freezer if so you have a defrost problem,

If the thermostat has been replaced, then I would take a look at the dampers between the refrigerator and the freezer. Here is why....
All the cooling takes place in the freezer section of the unit, there are passageways with dampers that connect the freezer section to the refrigerator section. If these dampers get stuck in a certain position you will either have not enough cooling or to much cooling. This sounds like it is your issue. Hope this helps.

Check to see if you can feel the inside fan. The fresh food section is cooled by the freezer only. If the fan is not running the fresh food section will be warm. Also make sure the damper is open between the freezer and the fresh food section. It is controlled by one of the two knobs inside the fresh food section. The knob that turns the unit off and on is the thermostat, the other is the damper control. They are sometimes confusing the way they are named by factory.
One of these two things should be your problem
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If, the fresh food section is 34- 38 degrees, then the freezer should be cold enough to keep ice cream solid. All of the cooling capability comes from the freezer section. I, personally, would not waste time or money on a ten year old box. Ten years ago it cost $1000, so, the outlay is $100 per year. Dump the old refrigerator and get a new one.

The fan noise has two reasons: one is that ice forms around them causing strain on the disk brush in the middle, which wares down the brush causing the fan to go out of balance, this causing the loud whinning and sometimes high pitched squilling. This happens when the unit is six months to a year old. The first thing that is suggested by the manufacturer is to let the unit be unplugged for twenty-four hours, allowing the ice to melt from around the fans. It has been my experience that this is only a short term fix. You can repeat this every other month until the revolutions per minute of the fans is so reduced that they no longer supply enough air ciculation to cool the refrigerator or the freezer. The solution to this problem is to replace these fans about once a year. Unless the noise does not bother you.

if there is frost build up on rear wall of freezer section the unit is not going through defrost cycle which builds up frost on evaporator and restricts cold air flow to the fresh food section. one of three things are bad. defrost heater, defrost thermostat or defrost timer. if no frost on rear wall you might be low of freon.

Your automatic defroster coil is open and needs to be replaced along with the fan in the freezer that blows the cold air around and also blows air into refrigerator.

Bad smell down below is food rotting in that section because it is not cold enough.

Hello. You need a new cabinet thermistor and while you are at it replace the evaporator thermistor. (The evaporator is the cooling coil behind the back wall of your fresh food section) . These sense the temperature in your frig. Both of them are behind the back wall of your fresh food compartment. You will need a wire crimper and a heat gun or lighter to heat shrink the wire connectors that come with the new ones.

It´s probably overloaded. Any refrigerator has a specific cooling capacity eg x Kg/24hours. you must check that capacity and charge acording to that.

This sounds like the defrost heater is bad or dead. The refrigerator will not cool because frost is building up on the evaporator coils in the freezer section and the fan can no longer push air past the coils. You will find that the freezer will build up with frost.
If you are brave enough, remove the cover in the freezer and you will see the coils are completely covered with frost (ICE). At this point with the refrigerator unplugged ( remove food). get a hair dryer or heat gun and melt all the frost. You will notice a heating element ( looks like a tubed light) -element will looked burnt out.
Replacing it at this point is easy. Before you start though, try to get the manual for your system and call around for a local appliance parts store to see if the part is readily available.