I have the 50g rubbermade on wheels for mixing saltwater. Love it. Was going to buy the vertical 65g tank from usplastic, but the shipping was a killer (group buy?). Rubbermaid also makes a 96 g which is pretty great. Both at home depot.
I use an old 75g that I mounted up high for storing do/di. I have the overflow setup to drain into the sink below if I forget to turn off the ro/di.
Just siphon down 50g, drop in salt, and a mag 12 warms the water to about 80 degrees while it mixes.
I plumbed a "t" off my return pump and fittted it with a quick connect hose fitting. Plug in a hose and dump 50g's in about 5 minutes. Then the hose clicks onto the mag and the new water gets pumped in.
Takes 15 minutes tops for 50g change. I have been doing it weekly, but will scale back once my fuges really get going.