Depending if you need keyboard pass-through or not, you might also like to look at this thread.
Photon - USB Host Mode
If you don’t this might solve the USB side of things.
For prototyping the PS/2 side you’d just use something like this
http://www.alibaba.com/product-detail/USB-panel-mount-to-Husoing-4Pin_349032128.html
And for SD card something like this
http://store.hardwarebreakout.com/index.php?route=product/product&product_id=59
The rest is software.