Hi! I made a Traktor controller myself and I used 64 buttons, which I think is more than enough to control pretty much everything in Traktor (that's what modifiers are for, and with led lights to remind you what modifiers are on its very comfortable) . If you plan to use all 4 decks, I would recommend you to make something like 2x NI X1 My link and a mixer section. Try to minimize your designs and maximize peformance. Planing something carefuly is much better than doing it twice.
Good luck with your project!