Something to be aware of re the older Proto2000 locomotives...
These are all "new-old-stock". LifeLike (who was the original manufacturer of the "Proto2000" lineup) made their last engines around 2004-2005 at which time they sold the business to Walthers.
So... you're likely going to have to go to ebay (or other used sources) to find them.
Some will be essentially "new in the box" -- never actually run (but still old).
Others will be "used, but excellent" -- perhaps pre-owned with little or no use, again, never taken out of the box.
They're almost all at least 15-20+ years old.
But don't be put off by that.
Be aware that a number of early Proto2000 engines had a problem with cracking gears on the wheel axles. This requires that you replace the gears (full wheelsets with good gears are available cheap) -- you have to pop off the "bottom plate" on the trucks, and put the new gears in. It may also be necessary to clean out some of the old grease inside (which looks like peanut butter).
But once cleaned up, these engines can run VERY well.
Two of my best "runners" are old Proto2000 GP's that I had to break down and clean up. But once reassembled -- smooth and quiet!