Ken, The Proto 2000 line was introduced in 1996 by Life-Like. It was produced by Life-Like until 2005, when the Life-Like brand and train business was purchased from Lifoam, LLC, the parent company of Life-Like.
All of the early (pre-2002) P2K models were not DCC ready and need to have the motors isolated from the frames as well as having the decoder hardwired. After that, things get murky. If you look at the proof of purchase tag on the back, it seems that most of the models that begin with "21" after the "77071" do not have DCC plugs although some have isolated motors and are fairly easy to convert to DCC. You need to measure voltage to the frame with an ohmmeter to know for sure. As usual, this isn't always true.
I have an SD9 with a "21" number that does have a DCC plug even though it doesn't say so anywhere on the box and and only refers to it in the last page of the instructions.
All models that I know of with a "23" are equipped with a DCC plug although you normally have to replace the 1.5 volt headlight bulbs with 12 volts bulbs or add dropping resistors to keep the original lights. Some DCC decoders come with the resistors in place.
After Life-Like was purchased by Walthers, almost all the P2K line were produced with DCC plugs and compatible headlight bulbs so all you have to do is unplug the dummy plug from the board and plug in the DCC decoder. There were some leftovers from Life-Like still in the pipeline when Walthers took over and those still need the headlight changes. Walthers technical support people are really good and can tell you if you have one of these. If it comes in a gray box with the Walthers name on the back and no reference to Life-Like, it does have a DCC plug and says so on the box.
All the early P2K BL2, GP18, and GP20, and Alco FA-2 had problems with cracked axle gears. If you notice problems with the locomotive running poorly with a grinding noise, it's likely caused by cracked gears. Walthers will send you a new set for free if you give them a call.
I don't know much about the Atlas engines or the Stewart engines in terms of DCC readiness. I'd call Tony's Train Exchange at 800-978-3472 and talk to one of their techs. They know just about everything about any locomotive model produced and what decoder is needed for each one. If you buy ten or more decoders at the same time, you also get a nice discount.