Nitrogen.....NOT compressed air. The idea is to rid the system of any moisture. After purging the lines with nitrogen, you'll need to hook up a vacuum pump and pump the system down. Under a good vacuum, any moisture in the system will be boiled out as you have lowered the boiling point of water via vacuum. Pressurizing with nitrogen before hand will show up any leakers that might otherwise loose all of your expensive refrigerant over time.
The expansion valve would probably not need to be replaced, unless trouble beforehand indicated so. The old 'o' rings should be replaced while you're at an A/C rebuild, the system must be leak-tight! The rule is, any time a refrigerant system is opened you should replace the dryer. I've seen some who didn't, but I don't have that type of luck...=)
There are two major brands of compressors, I believe Bill and winfred stand by the re-manned Nippondenso???