So I'm thinking of buying some liquid calcium if I get a snail, but should I be buying the liquid iodine too? What does iodine do for snails?

Iodine is for overall shell health/strength.

http://www.applesnail.net/

If you go to the link above and look at the green box on the left find the word 'Discussion' and it will take you to the Snail forum.

I'm new to snail keeping and have really hard water/high ph so I haven't gone crazy with calcium or iodine, I do add it a few times a week rather then every day.

You just have to be careful not overusing it. A couple times a week ,a couple drops,and you're good.