Not sure how many buttons palyers 3/4 need. I always thought that 4 were enough from what I remember. I don't think any 4 player cabs had the need for 6 buttons.
Anyway, I would recommend 7 buttons on players 1/2 so you can get the actual Neo Geo layout.
If money is super tight, you could just do the CP as someone hinted. It's not as finished but it will save you alot on wood, paint, screws, glue and time. You can also get a CP done without a router except for maybe the top piece depending on the shape.
My one regret with my first build was trying to build a "do as much as possible cab". i wanted it to emulate everything. N64 a primary emulator but that is a waste of time and effort in hindsight. I wish I did a 4 player cab and ignored analog altogether. I have a plan to do another build and I think it will be a 4 player cab. I might get a trackball on it also. That is TBD. Build 3 would be a driving cab with maybe a light gun.
My 4 player cab is going to feature 2 Mag Stick Pluses:
http://www.ultimarc.com/controls.htmlFor players 3/4 I might do a lesser stick but to get the same feel out of those sticks, I think I'll just get the same thing for players 3/4 even if it is somewhat wasteful.
As for budget:Estimate all your costs. In the end a full cab is going to cost you well over $500. Mine was estimated at $700 and probably closer to $800. Saving $20 on sticks here and $5 on buttons there is not that big of a deal when looking at the total cost. I go by the thought process of, spend the extra 5% now so you don't say "what if" later or have to deal with an upgrade later.
One cost saver I made:I wanted to get custom graphics for my CP. But that requires that you design graphics for your CP And after that, you need to pay for them. This is easily a $50 ordeal if you do the artwork on your own. I went to a local fabric store and got some textured vinyl for $7. Looks nice with it and hte texture makes it looks a little more interesting than paint alone. Or you could just paint the top of the CP.
Surprise:Plexioglass! That stuff is not cheap. It was 10 years ago. Not anymore. I spent about $100 on the stuff.