If you plan on staging a few cars that will not be moved frequently (i.e., cars parked on an industry spur or siding) and you are not a rivet counter -- I am definitely not -- you can get away with purchasing less expensive rolling stock. Truck/wheel/coupler upgrades are optional, though you might want all of your cars to have the same type of coupler just in case. While the inexpensive but easily upgradeable Athearn Blue Box kits of yesteryear have been discontinued, you can still find them if you're willing to check around. Check out the "close-out" bin at your LHS or search online sites, and not just for Athearn Blue Box kits. It's amazing what you can sometimes find on clearance. Accurail also offers some inexpensive kits that are decent and easy to assemble. In addition, Accurail offers runs of 3, 6, 12 (and more) different car numbers for some of their kits, which is nice. Finally, you can also do quite a bit of inexpensive "upgrading" (detailing) to a cheap-looking car with appropriate paints/coatings/chalks/decals.
For rolling stock that will be moved about the layout frequently, I recommend making sure the cars either come equipped with or are upgraded to better trucks/wheels (RP25), couplers, and (if necessary -- but usually not needed) proper weights. Using a computer analogy, you might be able to play a computer game using Intel GMA 4500 (integrated graphics) on an 17" monitor at 800x600 @20fps with medium settings but you'll enjoy the experience much more if you have an AMD HD5870 graphics card and can max out the eye candy on a 24" monitor (okay, 3 of them using Eyefinity) at 1920 x 1080 @60fps.
IMO, Atlas Trainman, Accuready and Bachmann Silver Series (excluding the log cars) are pretty good in terms of bang for the buck ready-to-run rolling stock. You should be able to buy most freight cars from the above lines for well under $20 per car, often $10 to $15. BTW, the Bachmann Silver Series log cars are very nice -- I have several of them -- but the list price is @$43 per car (c'mon, Bachmann -- help me out!). When Walthers runs a sale on its branded rolling stock (Platinum/Gold Line/Proto 2K), that's usually worth taking a look. I really like Atlas cars (and their trucks), too - probably my favorite overall, but a bit more expensive on average than some of the other vendor offerings.
At the end of the day, it's your layout and you make the rules (and set the budget). Everyone has their favorites, but in the end we'll end up with similar cars for a given road name from multiple vendors just for the varying car numbers and different detailing.
For rolling stock that will be moved about the layout frequently, I recommend making sure the cars either come equipped with or are upgraded to better trucks/wheels (RP25), couplers, and (if necessary -- but usually not needed) proper weights. Using a computer analogy, you might be able to play a computer game using Intel GMA 4500 (integrated graphics) on an 17" monitor at 800x600 @20fps with medium settings but you'll enjoy the experience much more if you have an AMD HD5870 graphics card and can max out the eye candy on a 24" monitor (okay, 3 of them using Eyefinity) at 1920 x 1080 @60fps.
IMO, Atlas Trainman, Accuready and Bachmann Silver Series (excluding the log cars) are pretty good in terms of bang for the buck ready-to-run rolling stock. You should be able to buy most freight cars from the above lines for well under $20 per car, often $10 to $15. BTW, the Bachmann Silver Series log cars are very nice -- I have several of them -- but the list price is @$43 per car (c'mon, Bachmann -- help me out!). When Walthers runs a sale on its branded rolling stock (Platinum/Gold Line/Proto 2K), that's usually worth taking a look. I really like Atlas cars (and their trucks), too - probably my favorite overall, but a bit more expensive on average than some of the other vendor offerings.
At the end of the day, it's your layout and you make the rules (and set the budget). Everyone has their favorites, but in the end we'll end up with similar cars for a given road name from multiple vendors just for the varying car numbers and different detailing.