Let's back up! What is wrong with the a/c to begin with? That will determine your course of action.

As far as a vac pump goes, I read that you can leave a deposit at AutoZone and they will loan you one. Sounds like a great deal for the DIYer.

All kinds of things are used to flush an a/c system. What is most economical is to initially flush components with mineral spirits then follow up with regular a/c flush to remove the mineral spirits residue.

I would not try using a venturi pump with a small compressor. You need to draw a vacuum for an extended period of time. You will have to have a really big compressor to be able to maintain a vac on the venturi long enough to adequately boil off moisture.

Good luck,