Definitely you need to get a good fire barrier as soon as you can, but that goes with any foam. My biggest problem with styrofoam is that it's not very consistent (which is why it's cheaper) and it's hard to cut and get a clean edge which makes it hard to get a good seal between the sheets. Work with some polyiso (which has almost twice the R value) and you'll see the difference. That said, you've got it and you got a great price so go for it, but plan on putting a sheet plastic vapor barrier on the inside and then cover it with a fire barrier. You can't leave the plastic or foam exposed permanently. That's just too dangerous.
We've talked about the inside finishing before and drywall, OSB, and metal liner panel all have their advantages. If you're not wired or adequately wired already, think about doing all your wiring in conduit on the wall surface. It will make fitting the foam panels a lot easier and give you a better seal. I learned this the hard way.