The following is just my own personal opinion as my tastes and interests have changed. Please, no offense to anyone intended!
'Collecting' MPC stuff seems more like something that would be called 'rounding up', 'gathering', or similar. There are a few truly 'rare' items produced during that era but from what I remember at train shows 10 -15 years ago there were tables with vast stretches of it everywhere. Probably hundreds of thousands of the common boxcars, gondolas, and tankers that had never been out of their boxes, yet they had a lot of miles on 'em! There are still sets in the original shrink wrap out there - and for what? These are toys, not 'valuables'.
Unless a lot of them have been thrown out by disgruntled spouses, most of those trains produced are still around in people's basements, under furniture, in closets, storage buildings, and the like. If you are patient, you can probably amass quite a number of boxes easily.
The MPC era certainly has somme appeal but unlike pre/postwar trains that have historical/antique value and the modern era of scale O gauge equipment with operational features, MPC is an unusual middle ground period before the better quality PWC reissues and CC reissues of the postwar trains...