First of all if you want to do this job right use R12, you will be doing the same amount of work to do it right. You can go online and take a test for $15 for 609 certification that allows you to legally purchase R12.
Blowing out the lines with compressed air will accomplish nothing, in fact it will be a negative because you will be putting moisture in the lines that will be soaked up by the oil inside. You will have the lines broken anyway, so flush it with brake cleaner if you don't want to buy a flushing cannister. THEN BLOW THE FLUSHING AGENT OUT OF EVERYTHING COMPLETELY.
If you don't want to mess with an old refrigerator compressor, which would work very well by the way, you can buy a small vac pump from ackits or somewhere for about $200.
As said earlier ONLY use Ester oil in a converted system.