2 8-way joysticks and 3 buttons per stick will get you playing most of the non fighters. For the classics, you would want to add a 4-way stick. It is not very fun trying to play Pacman with an 8-way stick.
If you ever want to go crazy and do the modular thing, give me a call. I have one, and I can let you know all of the pros and cons. In a nutshell: Pro - Play any game with original controls. Con - Lots of work, and you have to buy all of those controls. 100% worth it for me.
Let me know if you need any help whichever way you go on this. My suggestion would be to start with the easy one; then if you want more, expand it to modular, or make another one.