I recently got a Sharp Twin Famicom for around 70€. This is rather cheap, they usually go for over 200€ in good condition. The Twin Famicom is a Famicom with a built-in Famicom Disk System drive. This was a floppy disk system for the Famicom that was only used in Japan. The one I got worked well with carts (at least with 8BIT MUSIC POWER FINAL), but disks wouldn’t read at all.

If you want to get a Neo Geo, you should probably get the MVS (arcade) model instead of the AES (home) model. The games are much cheaper and 200-in-1 type cartridges are readily available. If you have an MVS board, you’ll need a way to power it and a way to connect controls and the screen.

As the Neo Geo MVS uses the JAMMA arcade connector standard, there are many ready-made solutions that will give you standard SCART RGB output, some controller connectors and a way to connect a power supply (either ATX or special arcade ones). I got one from http://www.retroelectronik.com. You can reuse this for non Neo Geo arcade boards, too.

Here’s the system how it was when I got it.

The PAL Mega Drive can rather easily be modded with a switch that enables switching between PAL, NTSC-J and NTSC-US. It doesn’t require much skill, if you can do basic soldering, this one is for you. For proper instructions, take a look at this site.

Here’s a picture of my Mega Drive running at 50hz.