I just got mine today, except a single DI, and it came with a 3g tank.
I would DEFINATELY go with one that comes with a tank. I can already see that's great to grab a gallon of top-off water. Without, you'll be waiting 15-20 minutes for a gallon.
Took 10 minutes to install once I made a trip to lowe's to get a new coldwater feed line (my old copper tubing was worn out).
I got the 3g tank and the faucet.
I've already made about 10g with it, after flusing the first 3.
I have read that dual DI isn't needed - 0ppm is 0ppm, you can't get any cleaner that that.
I got the add-on digital dual TDS meter, which shows in/out levels - from 295 or so down to 0ppm with a single DI.
I supposed the second DI would be handy if your first DI stage started to channel or get worn down.
I already see a need for the float valve, and will be ordering that later tonight. I can drain directly into a 5g bucket that I get my sea salt in, and have it stop when its full.