If you are already completely out of R-12 freon, you probably would be fine just. I usually replace the accumulator (cheap) and o-rings (cheaper still). You can get a spray can of flush to do your system. Then convert the fittings and add the required amount of oil and R-134a freon. If you still have a charge of R-12, you need to get it recovered. While they are there see how much it would cost to get your system flushed while they are in there. The last time I had a partially filled R-12 evacuated, I got the system evacuated and flushed for only 50 bucks. Considering the flush will run around 10 bucks, it's nearly worth the money to just get it done at the same time.
As for vacuumed, you need to get that done. If you have a air compressor, you can pick up a cheap venturi vacuum pump from Harbor Freight for around 9 dollars. Otherwise, you'll have to get that done professionally as well.