Consider the games you are going to play most, and then figure what you can live with, and what you can live without. Here are some examples:
1) If you are running most old school games, most can be run with 3 buttons per player.
2) Neo Geo games need 4 buttons per player.
3) If you like the newer fighting games (i.e. Street Fighter, Marvel vs. Capcom, Mortal Kombat Series, etc), you will need at least 6 buttons per player.
4) Games like Marble Madness, Capcom Bowling, Centipede, Missile Command, Golden Tee, Rampart, etc all need a trackball for input. Some of these even used dual trackballs.
5) Games like Arkanoid and Tempest used spinners for original input.
6) If you like Shooting Games, you may want to get one of the Act Labs USB Light Guns. These are the only guns that work with MAME well.
7) If you like Driving Games such as Pole Position, Hard Drivin, Out Run, etc, these games used a wheel and pedals. For these to work in MAME, you may need a racing wheel and pedals that can emulate joystick inputs.
Many flight sim games used an Analog Joystick, instead of digital. This joystick can input a much wider range of values for each direction, than the usual 8 positions.
9) Some games like Stun Runner, Star Wars, Starblade, and others used a yoke. These can be wired through Analog inputs.