I've done my share of hacking inside the Genesis, and your chances of getting a GOOD model 1 Genesis are (from my experience) not too great.
The one that's of better quality is the revision 1, or 2 (not model 1 or 2) But there are bad ones within that as well.
Firstly, you want one with the words "high definition graphics" on the top of it. Beyond that, you have top open it up and look at the CPU (the large chip behind the cart slot) if it says "MC68000" or some variation of it, it's a good one. if it reads "HD68HC000" it's also good. but the very early revision 1 consoles (the ones with the EXT port on the back) are known for using the "SCN68000" CPU, which is a bad one.
I have two model 1 consoles, and have checked two others, I've come across two SCN68000 consoles, and two Rev. 3 consoles.
Honestly, unless you plan to do a whole bunch of hacking with it, you're fine with the model 3 if you want to use the 32x and Sega CD, just get a model 2 console, they look better on it anyway.
If any of this didn't make sense, feel free to ask for specific information.
