If you are really talking about a 50ft long shelf railroad with multiple operators, walk-around control is a must. If there are more than 2 operators at a time, you are pretty much past the "break-even" point for DCC. The extra wiring, switches, multiple control panels, and operating headaches of cab control in your situation make DCC a natural.
There is an HO modular system called "Free-mo" that appears to fit in quite well with your stated plans. Here is a link to their forum,
http://groups.yahoo.com/group/Free-mo/, and I believe their module standards are in the files section. At least there are links to their standards page. Free-mo uses Digitrax DCC as standard, and they have the wiring and connections all included in their standards. All the engineering is already thought through for you. The added benefit is that there is probably a Free-mo group in your area, with whom you could team to display some of your modules on occasion. Even though I am not a Free-mo follower personally, I lurk on their forum to steal ideas shamelessly.
My personal preference is for integrated (baseboard, backdrop, upper functional bookshelf with lighting under) bookshelf layout sections that can be wall mounted in the living room. But because I'm different, I get to engineer my way through the anticipated and unanticipated problems myself - an aspect of the hobby I enjoy.
Another advantage of DCC is that you can program the performance curves of the locomotive into the decoder - setting top, minimum, and all the in between speeds. You can preset momentum effects. Momentum, when used, will simulate the coasting effect electronically. To do this easily, you need a DCC system with a computer interface, and a computer running JMRI (free software). Momentum and voltage presests (top and low) can be done with fancier throttles in straight DC, but would have to be adjusted when you switched locomotives.
The DCC auto-reversers sense the instantaneous polarity mismatch as the metal wheels of the train cross into/out of the isolated reversing section, and reverse the polarity of the reversing section before the system circuit breakers can trip. Apparently, the solid state units from Tony's are really sweet. You can do it manually - in
DCC you swap the polarity of the
reversing section while the train is in it. In DC, you change the polarity of the rest of the layout while the train is in the reversing section.
Couldn't tell you about actual or relative prices of electronic components - I haven't bought any in a while. At Radio Shark - if they still stock parts - diodes and resistors should come around 4 to 8 in a package for a few bucks (rip off price). Full wave rectifiers are a couple of bucks each (again Radio Shark's rip off pricing). But Radio Shark is getting out of stocking parts; you may have to order on-line. Out West, we still have Frey's Electronics which has some parts at reasonable prices.
A potentiometer (pot) and rheostat are the same thing, with the rheostat generally considered to be a higher power device. Both almost always have 3 connections - one at each end of the resistor, and one for the sliding wiper. In non-transistor circuits, usually only 2 connections are used to get the variable resistance. Pots in transistor amplifier or op amp circuits often use all 3 connections in a voltage divider configuration to set the voltage going to the transistor base or control gain of the op amp.
I empathize with you about the old Lionel stuff being able to be pushed by hand. Even at the age of 7, I enjoyed pushing it myself as much as I did controlling it with a transformer. It is the price the scale modeler pays for slow speed performance. To get the needed gear reduction single thread worms are used. This gear configuration cannot be reverse driven, which is why most HO cannot be pushed. A mulitple thread worm can be reverse driven, as can spur and helical gears. Lionel used multiple thread worms in some of its early large steamers, but the gear reduction is directly related to the number of threads on the worm. A single thread worm with a 36 tooth worm gear has a 36:1 reduction. A triple-threaded worm with the same worm gear has a 12:1 reduction. But kids used to race Lionel engines - you don't see that in HO. Because of the inability to be reverse-driven, HO locomotives don't speed up as much going downhill as Lionel, especially with a transistor throttle or DCC. They may "buck" if there is play in the gear mechanism or motor shaft and the train is pushing them from behind.