Need help quickly! Should I replace my furnace as well as my ac unit?

Our compressor died and our furnace guy, who we've had work for us for 20 years, gave us three options: 1) replace the compressor for $1200, which he does not recommend; 2) replace just the ac unit for $2150; or, 3) replace furnace and ac for $5150. It is all 20 years old, but the furnace has been working fine. He says our ac is a 9 Seer and the new one would be 2 ton at a 13 Seer. We have about 1000 sq. ft. My understanding is that we don't have to do it all to get the $1500 tax credit, that the ac alone would qualify. Also, we may want to move in a couple of years if the market improves. Would we be wasting money or helping to sell the house in the future? I've ruled out replacing just the compressor, but should we do it all? He says we have a congested set-up down there and it would be easier for him to do it all at once, but he's really not pushing. We've known him for 20 years and he's always been honest and sometimes not even charged for small things. He can do it as soon as I let him know. Thanks.