You will need obviously a Naomi motherboard and either some cart games or GD ROM games. Some were released on both, others just on one format. If you do get GD-ROM games, you will need a dimm board that plugs into the cart slot and a GD ROM drive. Probably cheaper to get them all together.
Beyond that you will need a VGA capable monitor (640x480), a power supply ( you can find these on ebay now and again, or you can build your own with a PC PSU and some plugs), a Naomi I/O board which plugs into the Naomi's USB socket and then a joystick to hack onto the I/O. Finally you will need an amplifier for the sound. Any normal amp or powered speakers with phono inputs will do.
It's not a massive undertaking, but you will need to do some soldering and a sacrificial stick/pad.
Giz10p is a great place to get Naomi parts, though his prices on some parts are a bit steep. For wiring he's a safe bet. For the rest I'd wait for it to pop up on ebay.
Save for that, I know you don't want to get a cab but Naomis often pop up on ebay for cheap with a game in there. If you get a GD-ROM game you are set, just rip out what you need and sell the cab.
Here's a great place for just about anything you want to know:
http://www.sega-naomi.com/
If you get stuck give me a shout
BTW, totally OT but were you interested in selling that busted 3DO?
