Arnold...here are a few answers to your questions. NOTE...some of your questions like "quality" etc depend on your point of view. I am a simple operator. Stop,Go, Reverse, lights, smoke, and reliable operation make "quality" to me. If you are into extensive electronics, crew sounds, special steam effects you won't find them in MPC (after all it stops in 1985).
a. MPC era...1970-1985
b. I don't agree. Much of MPC especially the early versions is really later Lionel post war but with "fast angle" wheels that makes the pull load much lower. Graphics all around are top notch, prototypical dimensions not so much. Yes there is a lot of plastic but then there is a lot of great molded in detail and the plastic used (at least in my view) seems quite durable. So, my view, like any product line you sort of get what you pay for. Lionel in those days made some dc only very cheap trains (sort of follow on to the "Scout" idea) and they were not very good. But their GP's, larger steamers, early diesels were quite good.
c. The answer to this is sort of "it depends"...remember Lionel in the 1950's was at the peak of its popularity and developed much of its best line...the 736 for example, dual motored F-3's, etc. MPC took quite awhile to come up to speed and in the early days (say up to 1975) concentrated on the Lionel of the 1960's which was not the same. I personally think the quest for what may go to quality is over rated. The MPC I run on my small layout (which is about the same scale as yours) runs great, stops, goes, reverses and runs on and on. I like the great graphics and the ability to run short trains (can't run long ones anyway). So to me, that's high quality especially when compared to what I paid for most of it.
d. I really can't answer, MPC did make some really good steamers which are probably top of the line, but they are not my focus. I leave that to others. I like the diesels, switchers, and freight and passenger cars. The short passenger cars, equivalent to the Lionel 2400 type post war cars are really great and came in some neat color schemes. The diesels and trainmasters were very good and came in a large number of liveries. My humble opinion (ok so not so humble 
)is that the MPC line sort of matches the Lionel of the late 50's or early 60's except the choice of RR and liveries is much greater and the color schemes available are much better.
Well anyway Arnold, that is my opinion and I am sure you can find those on both parts of the question. Its really a question of what you want to do and how much you can invest. I have small level layout , and like to see trains run around. I can't pull much more than 8 or so freight cars and maybe 5 passenger cars. I have some "Lion Chief" type engines but that is about as far as I got with non transformer control and normally rely on my ZW. I like the fancy steam sounds ... but made an inadvertent discovery recently...all those sound effects are in the tender! So now my MPC steamers (and my Marx steamers) sometimes pull a modern Lionel tender and I get all the steam sounds I want! I simply cannot invest in some $2000 for a new Lionel top of the line engine and I could not run it anyway. I do have (products of a more cash free youth) a MPC GS-4 and Hudson but they can't run on my layout anyway (radii too small). I have the MPC Blue Comet that I think of as sort of top of the line and it will run on my layout and I like it but that's about as elaborate as it go.
Best wishes, by the way, I always enjoy your posts and I really liked the series you did on constructing the baseball park.
Don
