In addition, Ultimarc also sells buttons and whatnot. I'd get everything from them in the interests of saving on shipping.
And I'll say get a J-PAC instead of an I-PAC. You'll need a JAMMA harness, but you can then wire your cab JAMMA compliant and if you get boards you can just pop them in.
I'm not sure a J-PAC would be the solution for him:
A) He is a beginner and that adds the more complex Jamma wiring to the mix
B) Even if he does get a JAMMA compliant board, the plug interface itself may be JAMMA compliant, but the pinout is different. i.e. Mortal Kombat vs. Mortal Kombat II - Both JAMMA, but different pinouts
C) The Mini-pac comes with a wiring harness to make things simple
D) He is aiming for an economical solution (he does not want to have to buy an additional Jamma harness)
The Mini-pac is the way to go in my opinion. Especially if he just wants to connect it to his PC to replace his keyboard controls with real joystick and buttons for MAME and emulators.
Good Luck!
~ DeLuSioNaL