Wow... Sounds like someone threw out a practical twin to my gaming A1200. Good find.
The MBX1200z board is memory+math co-processor (FPU) board. The FAST RAM speeds up the A1200 quite a bit. The math co-processor will help speed the unit some on highly math-intensive operations, but there aren't many things you'd run on an A1200 that you'd notice the difference with.
The board takes a single standard 72-pin SIMM module. If yours doesn't have one seated on it, I'd highly recommend adding one. (Like I said, the FAST RAM on this board is where it generates it's speed.) More information is available about this board on Amiga-hardware.com
MBX A1200zAs for a hard drive, I think you'll want to stay under 4GB, unless you update the kickstart to 3.1. (OS 3.9 has built-in support for larger hard drives, but you need to update the kickstart in order to have the minimum system requirements. You'd also need a CD ROM drive, which can easily be added by a Squirrel SCSI PCMCIA device.)
In short, if you want a small-end sysetem, stick to a HD smaller than 4GB. Going above that mark means needing a few extras. I think you might be able to get by without them, using some tools available on Aminet, but I don't know how well they work under the 3.0 kickstart. (And the problem is, if those tools fail, you can be faced with massive data loss, as there aren't many recovery tools...)