If you are looking to buy, you will need to invest in at least a 25hp screw compressor, delivering at least 100cfm, and a pressure pot. Anything less is a waste of money. It is best to find a good sandblaster locally, and develop a relationship with them. They need to understand your expectations, depth, etc., and they need to know enough to stop blasting if there are blow-offs.
We've tried both ways, and it is not worth the hassle to do it in house, unless you have NOTHING else to do. It is a long and tedious process, and you can make a lot more money making signs in the time it takes to sandblast.