Q: Would you put new engine & other possible new parts? OR get a new used car?

Bought used 02/2016 for $2000, put new transmission $2700 + $2100 for under the hood repairs, now needs expensive work done. Has been running great for 9 months - no complaints except for heater problem - takes 15 min for heat to get mildly warm; only blows mildly warm air out the vents when accelerator is depressed & car is moving; when stopped in idle, the blown air gets cold. Took to 2 repair shops for 2 diagnosis. 1st shop did block test, said it failed hydrocarbon test & very low on coolant (we added 2 cups after that), they recommend remove & replace cylinder head gasket, radiator hose, water pump, thermostat, do coolant system flush, said they found Stop Leak in radiator & it may have plugged heater core, possibly replace heater core. 2nd shop did diagnostic test on cold engine & said needs new engine, heater core, coolant flush. Also said they added gallons of fluid to cooling system. Two different recommendations, both expensive. What would you do? fix or get rid of it?

My car has 100000 miles.
My car has an automatic transmission.