$1600.00 Is a lot for a compressor, but not unheard of. I sure would
not put that kind of money in fixing a 20 year old unit. As for
replacing the outdoor unit only. ACs and heat pumps are sold as matched
sets. It is like putting a new Ford Tranny in a 20 year old Chevy. It
may run but probably not right. Also, having replaced many 10 to 15
year old systems, it is amazing how often the indoor coil is CLOGGED
with dirt and lint. Totally impacted. If you can afford a complete
system, go for it. As for efficiency, 13 SEER will soon be the low
end, many manufacturers are already out of their old low end stuff
(10-12 SEER). Try to get better than low end, ie 14 SEER or better. I
was just at a major trade show (Comfortech). It is amazing how cheap
some of the new 13 SEER stuff will be.

Get some one who does a load calculation and sizes the ducts properly
and measures air flow. Brand DOES matter, but installation matters
MORE.

Good Luck

Stretch