What the developer needs to know depends on how large the database is going to be. If the database isn't too large, your program could be written in Filemaker or REALbasic. If the database is large, the developer would have to know database programming plus knowing either Carbon, Cocoa, or Java. The developer would need to know C, C++, Java, or Objective C.
The amount of Mac programming skill the programmer needs depends on how quickly you want the program finished. A Windows programmer who had never written a Mac program would take longer to write your program than a skilled Mac programmer.
The ease in making a universal binary depends on the language used. REALbasic currently doesn't create universal binaries. New Carbon, Cocoa, and Java applications written with Xcode are relatively easy to create universal binaries. Reading data from files is the hardest part of supporting PowerPC and Intel due to byte ordering differences.