You could try Grantair Services at Marathon, I have been in there a couple of times for fuel and a stop-over. The people running the place seem very friendly and there is a flight school there. The students were cheerful as well. You may find Key West pricey, I don't know for sure but everything at Key West seems to be that way.
http://www.rentflorida.com/grantair/grantair.htm
http://www.floridakeysairport.com/
Edited to get the links to work!
[This message has been edited by Sensible (edited 12 May 2001).]