Good choice on the 360's.
As far as the 4 way stick goes, unless you're truly looking for the 'restricted' feel of a 4 way stick, the 360's are completely configurable to all 4 way games and will serve you well (not to mention just about any other configuration too, like Qbert, etc). If you're still looking for the restricted feel, just ask Andy at Ultimarc for a restrictor plate and the medium spring set. That will free up the real estate you need for a trackball.
As much you might think you may not need a trackball, you're really going to miss being able to play games that use it. For group sessions, Golden Tee was pretty much a must for me. If you're concerned about the amount of real estate it takes up, you can always get the a 2 1/4" ball setup.
As far as a keyboard encoder goes, get the minipac. It's really the best way to go in a 2 player setup. Not only will it handle all the buttons needed for up to 8 per player, but it will also handle your trackball inputs, AND a spinner. It will even drive a few LED's using the keyboard's capslock, numlock & scroll lock outputs.
I saw your other thread on the mini wheels. Because they're portable, I'd lean towards trying them. plus, the Turbo Twist 2 will fit in a standard button hole, which is nice.
Better add a USB hub to your list, if you're planning on running 360's spinners, and a minipac.