I've been making my own DDP's out of cassettes with no issues since 1986. The Sony HF60 was always my tape of choice (was already in use by a lot of the community by the time I learned how). To use one as a DDP, you simply need to drill two holes on the B side of the tape just under the write-protect tabs, then format it with the necessary audio track (I used a dual cassette deck and just copied an existing DDP).
Simply, I agree with your notion (the Coleco/ColecoVision/ADAM/Head to Head names Pretty much only this along with a few of their completely original games) that the name Coleco is great, but most of the games we knew them for weren't their own property, they were licensed.
A ton of these showed up several years ago and cheap ($15) from Jack Berg Sales. I was surprised to see how quickly they were snapped up since every CV sold had one included as best I could tell, so I didn't buy one. I figured it was a post-Adam release for those who bought the standalone Adam and never owned a ColecoVision. Now, I guess the novelty of it and the rarity make it sought after. I tried to find one for a couple of years before finally giving up as it wasn't worth more than $75 to me. You can still see the website listing at the link below, and they still appear to have some Colecovision & other classic console stuff, too.
To try and help find a barometer, can you guys post the serial numbers of the working and non-working CPUs? I think I read there was also a manufacture date visible inside the cartridge port on the motherboard that could be useful. That might at least start narrowing down which production runs are compatible without SGMs.
Man, I have quite a few fond memories of the 3DO-- the Gun Games were great (especially fond 2 player mode memories!)-- Mad Dog McRee and a few others. Of course, I was one of the pre-internet cheap-ass-gamers, so value went a long ways for me. Bought my Panasonic flip-top system at Best Buy new $89, and most games were only like $5 each on clearance. Pulled together a collection of about 50 games of so rather quickly. Standouts for me were Twisted, Gex, Daedalus Encounter, and while not a game, I was enthralled with Sid Meier's CPU Bach. I sol dmine years ago to finance something I wanted in my 20s, but worth a look in my opinion if you can find one rather cheap-- no clue what 3DO market pricing is these days.
For my two cents, I'd say keep it simple, and appeal to the largest possible audience with a modest base hardware.
I'd go minimalist, HDMI only, DB9 and/or USB controller ports, and an expansion or docking port for ala carte cartridge adapters at <$30 each. I liked Pixelboy's idea in another thread I think about using a dockable shape. Seagate did this with their Freeagent Media Player to allow their portable hard drives to fit the unit-- pretty slick design! And I think I first saw Bill suggest you can always offer a Zimba 4000, or something as technology and features are more feasible and sales history dictates the need.
If you can get this shipped for under $200 with SD compatibility for your cores, updateable cores for bugs/new additions and ala carte cartridge support, I think you have a winner. I'd buy 2!
Eccofonic: Not sure what to tell ya, especially in light of Jaymeister's post. I was alerted to the price by a friend who is a manager of one of the local stores, so I assumed it was B&M, too. But I do see it's $20 online. Grab one of each and get your free shipping from the dotcom if they'er still $40 locally. For me at $40, it's an OK buy and an easy way to get some Colecovision titles I didn't have (IE Homebrews). At $20, I'd say it's a no-brainer even if you do have to wait for shipping.