I'm your height and I also ride 18" in an XC hardtail frame. As far as bike companies that go by small, medium, large, etc., most of the time I fit a size Large, but occasionally it's size Medium that's right for me. You can't know without trying it out. Actually, you can know if you go to Marin's site and compare frame geometry to yours. You might have to convert inches to metric or vice versa. I'll leave that task to you.
As far as it being a good deal, I'd say if it was barely used and $100 cheaper it would be a good deal.
I like the fork. Heavy, but solid. Good luck.