Or standover height, from OP's description. In answer to your questions about size and suitability for fixed, OP,

How tall are you? I'd say a 52cm frame for you if you have a 30/31" inseam is going to feel kind of cramped w/o a longer stem and a seatpost w/ setback. I'm 5'9" w/a 30" inseam, and 53cm is about the smallest I can ride; even that feels tiny.

As far as converting to fixed, yeah, you can easily convert a Shogun to fixed. And I agree w/ whoever else said $25 is a good price for any bike.

Oh, and if you're converting to fixed and switching over to 700c wheels, then you should be aware the frame's going to drop a bit more--4cm. So pedal strike might become an issue there, depending on the height of the bottom bracket. But if you're just planning on gluing a fixed cog onto the original hub, then don't worry about that.