A 20 year old A/C uses R-22 refrigerant. The new A/C units use R-410a refrigerant. A new condenser will not work with the old evaporator coil unless you get a "dry nitrogen" R-22 unit, which are very hard to find now. It's been my experience that by the time you pay for the refrigerant and higher cost on the R-22 unit, you're within a couple hundred dollars of the R-410a system. R-22 is harder to get and more expensive as the EPA is phasing it out, so future serviceability is an issue. The line set can be re-used. The furnace can also, it will NOT ruin it. However, being 20 years old, it is time to replace the furnace anyway, and you will save some installation cost by doing it at the same time as the A/C coil. If the line set is easily replaced, it's not a bad idea to do it too. Otherwise, it can be flushed out. Your insurance company should pay for all of the A/C unit (outdoor condenser and indoor evaporator coil), as technically there isn't a viable replacement for only the condenser. DO NOT let anyone tell you it's OK to put a R-410a condenser onto a R-22 evaporator coil.