Well, first let me thank you both, for your knowledgable and speedy replies. It's really great to know that there are people willing to share hard-won information about these machines so readily.
Otto, I never heard too much about the 3M machines, but from what I've read on your other thread about the restoration you're doing, you be the man to talk with about these. From what you've stated about the rarity, though, I might have quite a time getting a parts bin together, although there is the option of finding a kaput one and cannabalizing it. I'm going to do some research about the availability of them just to see if they would be a practical daily use recorder.
I have heard that the MCI machines are deficient in bass response, at least Jack Endino has some charts that seem to show a marked roll-off at under 40 cycles. He seems a trifle prejudiced, though, so this bears a further look as well. Blevin's is one of my regular stops when window shopping on the web.
I work on a narrow format 4 track right now, and I have to say that you're dead on right, leaves you no room to hide at all. It's also pretty limiting when trying to get stuff balanced properly, and it takes careful planning to bounce without getting a wall of mud instead of clear performance from these things. I understand that with wider format, I would have much less generation loss, but I still think that a 16 track machine would make things a lot easier, allowing me to concentrate on sound rather than planning track order.
CJ, trust me, I don't live for the moment I can solder. It's a royal pain, but I had to gain skills to do some of the stuff I do. I learned to fix automobiles solely because I had seen so many friends and relatives get reamed when geting their vehicles fixed, and with tape machines, it's actually hard to find people who will work on them close enough to avoid shipping. I doubt I'll be designing circuits anytime soon, but replacing old parts with new ones is pretty worthwhile to know how to do. You can't really go wrong if you are methodical and careful with the good irons made today.
Finally, if I can get by spending less than my budget on a fine used machine, I certainly won't complain. Tape is expensive, and my next step after this will probably be a really first rate mixer, so I can use a headstart on that. In fact, expect another one of these types of inquiries in about five more years on finding a good console...