A36 steel is common, run of the mill, generic steel. Any other specification is uncommon/specialty steel. Specialty steel will always cost more, it will never give you any qualities that you'll need, and could possibly utilize components that aren't safe for food use. A36 is always what you want to look for.
Now, we've had some members who've known metal workers and that's given them access to free or very low priced special alloys. If you've got a similar in, I wouldn't refuse a special alloy, but, like I said, I'd look at the components to make sure it's food safe. If you're getting an amazing deal on the A572, let me know and I'll research it's constituents.
As far as the sizing goes, I think you've gotten some good advice. The only thing I'd reiterate is that if you ever plan on being serious about NY style pizza, you'll want the largest plate possible and that you need gaps for air on the sides (horizontal), but not on the front. If you can fit an 18" (or larger) deep plate from the back wall to the door, measure it to the nearest 1/8". Keep in mind that you'll get some space in front of the shelf and to make sure to utilize it. If your shelf has a lip (many do these days), I highly recommend taking the route Mary Ann took by raising the plate above it. Measure carefully.
As far as the gaps on the sides go, it's about horizontal real estate vs. weight. I think Mary Ann is in the right ballpark with 20". I went from 17 x 21 soapstone to 17 x 17 steel, and, I have to admit, making sure I hit the lateral target is kind of a hassle. I'll probably get used to it over time, but, right now, since I'm concentrating so much on aligning the pie from back to front, adding the extra stress of the side to side placement isn't much fun. If you want to trim a little bit of the weight, 19" isn't the end of the world, but I think 20" is a pretty happy number for the side to side dimension.