Replacement?????

I have a York hvac unit… 25 years old..it is upstairs where 2 guest rooms are and seldom runs in winter due to heat rising from first floor cathedral ceiling. Never runs in winter unless outside temps fall below 10 degrees here in KY.. I have been up to where the unit is and inside still looks brand new in the burner areas and has new control panel. issue is the cooling side.. would it be reasonable to just replace the cooling side.. or would that be stupid… only about 1/3 the cost of the whole unit replacement????? big job to replace whole unit due to location…