Hi Mike......As Andy said, I think unit trains look very realistic. Sometimes the major manufacturers offer a "Train Set" that usually includes 4 or 5 similar cars, and in a lot of cases will offer a further "add on" set of 3 additional matching cars. I have a GTAX tank car train that was purchased that way. 5 cars with the set and a 3 car add on set. (see pic) Pretty much all of the major manufacturers produce really good looking, and realistic pieces, but as with everything, the really nice ones command the bigger dollar.
As far as passenger cars, there are a lot to choose from. My layout has 048 & 060 curves, so I opted to stay with the smaller sized passenger cars, as I think they look better. I think the smallest you can get are around 13". The other consideration is the construction of the passenger cars. Aluminum bodied cars seem to be the cream of the crop, but some of the prices for Aluminum passengers cars are well up there. I went for a set of Williams 15" (60 footers in the real world) Aluminum cars, which I'm very happy with, and In my opinion they are a decent price. For a similar Lionel product it would probably be almost double the price.
Personally, I'm not a "collector" and want to be able to run my stuff, so for us smaller layout guys, I think it is important that you decide early along so you don't end up with a mish mash of stuff that ends up on shelves or in boxes. I don't really consider my layout all that small, it's an "L" shape about 22' x 16', and I'm almost at my max as far as the number of pieces I can have on it at one time.
REV