• The Best Version of the Genesis - Buying guide on Retro RGB.
• GUIDE: Telling apart good Genesis 1s and Genesis 2s from bad ones. - Buying guide on the Sega-16 forum, has most hardware variants.
• https://consolemods.org/wiki/Genesis:Buying_Guide Buying Guide - Buying guide on Console Mods. Detailed, with lots of photos.

• I want the best audio chip.
• I want 60Hz.
• I want the best compatibility, but this is almost universal.
• I prefer the original design to the Mega Drive 2.
• I don't want to have to mod shit.
• Step-down aren't needed, can just buy AU AC adaptor.
• Not super fussed about video quality. All support RGB, some models I don't want anyway have jailbars.

The basic, foolproof, guide is: If you're buying a Genesis get a Model 1 with the EXP port on the back. If you're getting a Japanese Mega Drive get one with the text 'AV Intelligent Terminal' 'High Grade Multipurpose Use' on it. Don't buy a PAL one. If you want to fuss over details and try to get a bargain check the guides linked.