Go for a japanese machine if possible, the US one has censorship.
Avoid the PAL ones really, you can mod it to 60Hz NTSC quite easily, and you can also fit a universe BIOS to get around the censorship issues, and change the region from Euro/JP/US, and change it from AES/homecart mode to MVS/arcade mode too - as well as a lot of other things.
The system will only do mono through a normal RGB scart cable, if you want stereo you'll need to buy a different scart cable with a secondary headphone jack style lead that plugs into the headphone port on the front of the system.