I have a 13 yr old Bryant A/C- Gas Pac package unit. The top section that directs the flame into the heat exchanger is rusting out along with the choke bar on the bottom of the exchanger. The actual heat exchanger sections looks to be ok. A/C works fine. Have had few minor repairs over the last couple of years. Induction fan is starting to sing sometimes. I plan to live in the house in TN for a least 20 more years. I have several questions:
1. Would it be cost effective to repair such an older unit?
2. Would a heat pump w/resistive heat banks or a duel fuel unit be better where average daily winter temps range from the 40s to the teens?
3. I was looking at the Carrier Infinity 13 SEER and the American Standard DCY 12 SEER duel fuel package units. Are these good units and IYO is one better than the other?

Thanks.