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Depends. The new compressor may extend the life of your system for another 3-5 years. So you'll save some money in the short run, unless something else brakes (which is very likely if you AC is 15-20 yo) and you'll have to invest in repairs again and again. If you buy the whole new unit you'll pay more now but may be saving money in the next 10-15 years. More, there is this tax credit now that may save you up to $1500 off the cost of a new high efficiency AC (16SEER or better).

And it won't be easy to find someone who will charge your AC with the self installed compressor. They (HVAC guys) just hate this kind of job.

It depends. What's wrong with the compressor? Was it a burn? How old is the condensing unit? Which refrigerant? Yes, you could probably change the compressor yourself, but you'd need to recover the refrigerant first, which requires an EPA certification. Changing a compressor is not a DIY project.

I think you should buy a whole new system; Puron, with a 90 percent furnce, and have your ductwork resealed and rewrapped. Shouldnt cost you more than about 8 to 9 grand. Come on, go for it.