I'll throw my 2 cents in here based on my limited recent experience
Interfaces: it seems like my only "real" option is the iPac4, either the standard or ultimate. The KeyWiz40 just doesn't have enough inputs for 4P, unless I am missing something? I don't plan on using LEDs in any capacity, and I want to have 4P with a trackball. My best bet here is the iPac4 and a USB trackball, right?
I don't know the price off the top of my head on the USB interface for the trackball, and I didn't see you saying you want a spinner, so as far as I know, the extra $35 for the ultimate would only give you the trackball interface, but in using it you lose some of the player 4 inputs. You can make up for them with the 4 auxiliary inputs, but if you want a pause or service button, that is out too. I managed to get 32 buttons, 2 joysticks (2 are u360's so don't use the ipac), trackball and spinner on the ultimate, but I used every single input. No sense if you aren't going with LED lighting anywhere, you can do it all with the ipac4.
Trackballs: There are a few options out there, and the best deal appears to be from X-Arcade. Anyone have any experience with it directly? If there is a better option in terms of bang for your buck, I am all ears! I don't want to eBay special this build, or always go with the lowest price if that means I am sacrificing performance at every turn.
I had a tankstick first and thought the trackball was crap. Brand new out of the box it feels like it needs to be cleaned badly, like it has been used for a year or two. It is so bumpy and stuck on the left-right axis all the time. I went with the ultimarc u-trak on the new CP (I added led's and the translucent ball) and I like it. If you can cut a perfect 80mm hole you can go without the trim ring and it comes out really nice. But that is a tricky feat because it is a finished hole. The trim ring for it would only be useful if you were using 1/2" MDF or routed out the back of 3/4" to bring it up about 3/16", but if you use the trim ring you can just use a 3 1/4" hole saw.. it is a trade off on how you want it to look. I don't have any experience with other trackballs so I can't say more, but so far the u-trak is smooth as silk and looks fantastic.
Pushbuttons: I realize there is a lot of variance on opinion on what makes a good pushbutton. For the most part, I feel like fighters and shmups will dominate what we play outside of Golden Tee. I think I agree that I should buy one of each main style and see what I dig, but the Happ competition pushbutton is what we all know from streetfighter, correct? Is the Arcade PRIME pushbutton decent? If there is not much/no difference with the Happ comps, do you lean toward the more "traditional name"? If I decide to "upgrade" to the ultimarc offerings, is there a major difference in feel between the two, GoldLeaf and Classic?
All I can say about switches so far is that I am unimpressed with the GGG basic switches and the electric ice II switch mount is leaving a little to be desired. The buttons don't feel that "solid" to me. But they aren't all the different feeling from the tankstick buttons either. The only thing I can say about leaf switches is that they take up some room on the back side of the cp, so plan carefully for that. I could never have used leafs in my cp without moving buttons around a lot to accommodate for it, and as it was I had to trim off the leaf mounts on the GGG buttons. The Ultimarc basic buttons and switches feel much more solid, although maybe too tight of tolerances because one out of four that I added seems to stick just a little.
Joysticks: I feel that at this point there is no reason to get into the U360, strictly due to the price point. Even if I make changes to the interface and go with cheaper buttons, it comes nowhere close to offsetting the cost. I realize this is a highly subjective topic. On one hand the price of the Happs is favorable, half of the Sanwa's. But if I were to upgrade all 4 joysticks down the road, then the ~34 dollars would be a waste, right? I've read a lot of opinions on joysticks from the search results. When it comes to the GGG Smoothstick and LeafPro, would anyone recommend those OVER the Sanwa's? Why? I don't see the point of buying the JStik when I can purchase a legitimate Sanwa. Is there another Sanwa model I should be aware of? Any controllers not on my list that I should research?
I added j-stiks for my p3 and p4, and u360's for p1 and p2. I like the j-stik feel so far, although I have not played games with it yet (tonight maybe). The u-360 is smooth as silk but feels like it has too much throw even with the top mounted restrictors, so I am not sure I will like them. I might double up the springs to see if that helps. The cheapo sticks in the tankstick were too loose and undefined. They worked and felt good but in play it felt sloppy. I think I just need more defined corners. If I didn't care about flexibility, I would do the j-stik's personally. The 4-8 switch from underneath is a really nice feature, I like the feel of the stick in general, and it was a real toss up between the u360 and servo stik for the main joysticks.. The idea of not having to mess with anything to get true 4 or 8 way (ie with restrictors) was a big one for me.. but cost is a factor too, so given the choice and a stricter budget, anything that feels like the j-stik would work, even if I was stuck in 8-way mode.
Misc: Aside from some tools and bits, is there anything I am overlooking or forgetting for the CP? I plan on getting a drill guide, although my wife signed off on the drill press (yay!), forstner bits, PCB feet, and small things for wire management.
zip ties, zip tie mounts, a good crimper, some red and blue butt connectors, some extra wire (22 ga would be fine, 18 if you have to buy at a hardware store). extra female spade connectors, and a lot of patience. Might even be good to have a solder pen and some heat shrink handy if you run into any issues. Avoid the prewired harnesses if you can, they are crap. Have a multimeter on hand too, chances are you will have a bad connection somewhere (especially if you use prewired harnesses). If you are doing t-molding, have a 1/16th" slot cutting bit.. And overall, a router is your best friend. Flush trim bits to duplicate from a template, dado and rabbeting bits are good to have, and a roundover bit if you are doing any edges that won't have t-molding.
Even if you are doing graphics on the CP, prime and seal whatever wood you choose and sand to 220 or 320. The finished quality of everything is going to be very dependent on how much time you take to prep surfaces right. If you plan to paint with latex, remember that latex can take 2-4 weeks to fully cure and is very fragile until then. You could build a cabinet to perfection but if you finish it poorly it will look like you slapped it together with CD plywood and a jigsaw.
That's about all I have to offer here, good luck on it!