Been covered ad nauseum, but to make sure you don't start down the wrong path:
No standard MAME arcade game uses more than 6 buttons.
Some people add a 7th, so they can have the Neo Geo 4 in a row/properly placed Mortal Kombat 3 run button.
For the Neo Geo games, I've found that I prefer to use my thumb to hit the first button in the second row rather than having 4 buttons in a row.
By default, MAME assumes you have the following layout.
123
456
Lay out your buttons like that and most games will be mapped correctly by default.
I'm telling you this because some younger builders like putting 123 on the bottom.
You can do that, but you'll end up having to remap all the games that require more than 3 buttons.
Only two 4-player arcade games in MAME use more than 3 buttons per player. Those are the (2?) Dungeons and Dragons games and they use 4.
Guilty Gear Isuka on Demul emulator uses 5, but the game is generally considered the worst of the franchise.
Player 3 and 4 joysticks should be mounted square and not angled. You can angle the buttons so the player stands at an angle, but keep the joystick oriented the same way as player 1 and 2. Look at where the arrows point on original 4 player arcade machines and search the forum for "angled joysticks" for further explanation.

If using a CRT, I'd go with a 25-29" model. Bigger if building a showcase cab.
You didn't mention whether it would be an arcade monitor or television.
For TV:
If in Europe, look for one with a SCART input.
If in the Americas, look for one with a component input.
(component is three RCA connectors - green, blue, red. COMPOSITE is the single yellow connector. S-video is better than composite, but not as good as component)
You might have to use an older video card or an HDMI to composite converter since most modern video cards don't have these as output options.
The ipac 4 is a great input device. I'd recommend reprogramming the first four MAME default player 1 buttons to plain letter keys though.
I'm talking about reprogramming the ipac, not remapping them in MAME.
Do this before setting up your controls in MAME, then just map the first four Player 1 buttons in MAME "for all games".
It will save you the trouble keys like ALT and CTRL will cause on other emulators.
For PC games like Super Street Fighter IV and Mortal Kombat Komplete you can create virtual xbox360 controllers using the files stickied in the software forum.
The ipac will work fine with them.