Driptroit - pre and post emissions

I'm doing some research on used motors for my business. I don't know a whole lot about trucks and motors but from what I gather reading this forum any of these new fangled emission motors with EGR are worth more trouble than they're worth. From what I gather the CAT 3406 E is the old non-emission Cat motor and is great, while the C-15 is the popular newer CAT motor and sux in comparison.

With Cummins, the N14 is the older (and good) non emissions motor and these new fangled ISX and ISM are best left alone.

What I'm having a hard time figuing out is how you identify a Detroit pre and post emissions? I know the old motors are series 60 but don't they still make series 60? Also what's a DD detroit?

Also, are there any companies that still build motors without all the emissions crap?

Any help would be appreciated.