1) If you want to go all out, you could add a yoke, a steering wheel, and a joystick that supports more than eight directions.
2) When all else fails, design your own. Most four player games don't require more than three buttons per player, and for simplicity's sake, you may want to consider that when designing your panel.
3) You will find many different opinions on this, and as such you probably won't find concensus on what is the "best". Purists like an original monitor, some settle for a television, and others install an actual computer monitor. It all depends on what you want to do. Some people even go so far as installing a rotating arcade monitor so they can play games like Tempest and Dig Dug in their "true-to-arcade" style. I use the original Wells Gardner monitor that came with the cabinet with an ArcadeVGA card (an upgrade from a Matrox G400) and I am reasonably pleased with the result.
4) There are sites out there that offer compatibility lists for arcade monitors, but if you are using a television or a computer monitor, any decent card will do (just check the features of the card in order to make sure it does what you want it to do. If you use a television and buy a card with no S-Video out, well...)
5) I built my forst panel using a hacked Sidewinder, and I was happy with it. I don't see a problem with going the USB route. I use a J-PAC and I am happy with that, as well.
When I was building my cabinet, I visited every site Google would belch out on the subject and I found it interesting how many ways people came up with in designing their cabinets. Some control panels are so crowded with controls, it looks as though you'd need a doctorate in quantum physics in order to operate them. (This is not a put-down; I would love a cab like that in my home). I went a simpler route. I settled for a two joystick panel with seven buttons per player. The reason I did it this way was to make it easy to operate and for it to be as "user-friendly" as possible. I wanted to give friends and guests interested in playing Dig Dug simple instructions on how to operate the machine.
Just remember, design the cabinet for what you want to use it for. If you aren't planning on playing Tron or Front Line a great deal, maybe you can skip the spinner. It sounds like you already have your "dream machine" in mind, and a goal set-- Playing side-scrolling beat-em-ups. You probably wouldn't want to install a flight yoke or a steering wheel if you hate Final Lap and S.T.U.N. Runner.
Best of luck! This is a great forum, and if you can't find the answer here, you probably won't find it anywhere.
APf