Is it a wise choice to replace a compressor in an 11 year old Carrier 42k BTU heat pump rather than replace the whole outside unit?

No it's not. But if that's all you can afford then, you have to put the compressor in. You will have a one year warranty on the compressor. If you have a new system put in you can get a ten year warranty on every part. Goodmans and Amana, also have units with lifetime warranties.