Wow
Kick 
- a K2000 is a lot more than just an effects machine! I remember when that was the bees knees.
Did you buy it new or pick it up cheaply recently and ... what made you pick it up in the first place?
Those questions asked, back to MIDI interfaces:
Here's a list of MIDI to USB interfaces on audiomidi.com.
If it is possible to daisychain the K2000 into the back of the PF100 on a spare channel, then one input/output ought to be enough. However, if you want the possibility of separate input from each controller (presuming your K2000 is a keyboard and not a rack version) then a 2 input interface will probably suit you better in the long run. Check out the MOTU Fast Lane which is still available in a selection of colours, I think, ... if that floats your boat.
Adding audio capabilities bumps the price higher ... not sure of your budget - so have a poke around that audiomidi site to see what suits.