25 pecent C02 mix for a bit less splatter. I get mine at oxy pro. I buy my bottles outright, but you can rent.
My bottle is a small, and is good for about 2.5-3 hours of constant welding. That usually takes me a month to 2 months go through. Depending how busy I am.
For 1/4" steel .30 will do the trick and still be ok for smaller stuff. .35 is going to be a bit big on the little stuff, but works well on 1/4" and 3/16's.
A miller 210 is a nice welder that is big enough to handle all your thick hobby stuff, but can go low enough to still do sheet. Its also set up for aluminum spoolgun and dual bottles if you ever get into welding aluminum.
The 210 is rated to weld 3/8's single pass. I've honestly never needed mine set at more then a 4 for voltage, and it goes to 7. But Im slow and steady, I guess you could go higher if you like to burn things in really fast.
I definately recommend it for a solid machine around the garage.