"4 Player Buttons" - are you talking about the white start buttons with the little men on them? Don't forget about having credit buttons (I personally like having seperate start and credit buttons).
I also went for the 7 button layout but only for players 1 and 2, only had 6 for 3 and 4 despite what people have said. Standard mame only has 4 inputs for 3 and 4 at the moment so that's something to think about if your only going to use mame.
Make sure your centre joystick isn't in the way of the trackball. In a fair few games you need a clear area around so you don't bash your hand.
Check out
Bob Roberts for cheaper Happs products. Also check out Oscar's
1st Review and his
2nd Review of various joysticks. And while your there you should also have a good look at his fantastic spinners.
I went for Happs Optical Rotarys for players 1 and 2, and Happs Supers for 3 and 4. The rotary's are based on the supers so there's no real difference except perhaps a little extra weight on the bottom of the shaft. I went for Supers as they seem to be a very good all round 8-way stick. They are good enough for someone like me (a hack) that plays all different types of games.'
I also got some t-sticks from Andy which will go on another control panel
check them out. Also check out the Ipac's and Optipacs for the interface.
Good luck.
-cdbrown