well it depends on which trackball you buy...
I *assume* you are going to want to buy the ps/2 3" trackball so you won't need a separate encoder (like optipac from ultimarc, or pre hacked usb mouse from oscarcontrols)
I'd pick up a mounting plate.  Did you want flush mount or do you mind if there are carriage bolts showing on the top.  
Not sure if betson sells a "hardware" kit for the carriagebolts you'd need if you went with the "old school" mounting kit as opposed to the flush mount.
*shrug*  upon further review I don't think they sell a mounting plate (or at least not via their site that I could find)
this  IMP 47-4305-02   ADAPTER PLATE/ 3" TRACKBALL   $1.89   is NOT a trackball mounting plate from what I understand.  
If you did end up having to go 2 vendors which it looks like you might... why not get the accessories/plate other stuff from 
http://www.therealbobroberts.com for better than happs prices and down home service. 
good luck!
Rampy