Mixing your own saltwater is a relativly easy process...Just go out and buy a rubbermaid container, about 30 - 40 gals, have your RO unit drip the water direct into that, when full, add your salt and mix..24 - 48 hours before you want to use it, switch on a powerhead and heater to aireate and your good to go..Nice and easy saltwater...Plus, the bonus of doing your own, is that you always have mixed saltwater on hand if you ever need to do emergency water changes out of shop hours etc etc..
On a tank of 120 gal, i wonder if just a powerfull pump is going to be enough? its a large body of water in there, and there really does need to have good circulation round the tank. A pump will circulate yes, but i dont think it would be enough just on its own and i think you will end up with dead spots where detrius will start to build up causing problems down the road..Powerheads, like the maxijet 1200 dont really cost that much and they are a great way to get good turn over in the tank..
Hope that helps
Niko