I can't answer most of your questions, take a look at the thread comparing the two. You will see randyT and Andy go into finer detail.
The 2p/4p is totally up to you. Do you have three friends? or just 1?
I have bought and Ipac4 - eggsalent and easy with great support software.
I have bought a keywiz eco - eggsalent but harder to wire up AND NOW HAS SOFTWARE
both come from great reliable folks
I have just bought 3x ipac2 for 3 friends projects because they are one of the easiest to use.
If the keywiz software was around I would have been tempted to get a keywiz (doh)
I personally would go for the ipac4 rather than doubling up on 2 ecos but to each their own.
with regards to the control panel software
http://www.mameroom.com/files/CPD_Setup.zipShould send you in the right direction
take a look here for info
http://www.retroblast.com/reviews/mrdesign.html