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Fleischmann trains are made in Germany and are of very high quality. Most of my model trains are British or American. But I do have a few Fleischmann locomotives in both HO and N gauge and they are really superior. They are comparatively expensive in the United States, but well worth the expense.
Kato is superior equipment, but they only manufacture American and Japanese models, not European as far as I know. Roco track I believe is nickel silver. I have only used their N gauge track, not their HO, and that was all nickel silver. Sounds as though you're on the right track.
Remember that even though tracks from different manufacturers may interchange different manufacturers use very different couplers. HO/OO trains need 12vDC that has speed control and a reversing switch. The 16vAC is only for accessories, lights, turnouts switches or points. Never attempt to connect your running tracks and locos to AC power -- it could could burn out the motors.
I do know that the Southern Crescent ran up to the time of Amtrak in the 1970's. I believe it ran on the Southern Railway before this company merged with the Norfolk & Western to become the Norfolk Southern. Probably by the 1950's or 60's it would have diesel locomotives, either E8's, PA1's, or DL109's. I would think there are cars and diesel locomotives in Southern Green and white. The Southern also had steam locomotives, 4-6-2s and 4-8-2s, painted in green, which could be used. Riva Rossi's products have had mixed reviews over the years.
I doubt any would really compare with modern equipment such as Proto 2000 or Ahearn Genesis. They probably would not have motors with 5 pole skewed armatures or flywheels. The detail would be plastic cast on rather than separately applied plastic or metal parts. But detail is a subjective issue. Many modelers are happy with poorer models that meet their prototypical needs. I fall into that category.
I have some early brass engines that don't compare in detail to recently issued plastic bodied locomotives. But the locomotive I like may not be available except as an early brass version. More modern models with finer detailing and better mechanisms will invariably cost more, even if made in China. For a more definitive answer I would contact the Southern Railway Historical Society with respect to whether models of Southern Locomotives and cars are accurate.
I took a look at the Riva Rossi cars in the Walther's listing. These are pretty generic, standard 1930's vintage heavyweight passenger cars, just painted in Southern Crescent colors. I doubt that they are truly prototypical. I would sense they are family reasonable running cars. Depending on the era your modeling they might be out of date, as I would sense that the Southern Crescent, as a premier passenger train, would have employed streamlined lightweight passenger cars early.
Snap track is okay but needs care in laying, as does any track system. Its main problem is a limited amount of different radii. Probably the best, and most expensive, track system of a snap track type is Kato's. It is not compatible with other makes. Atlas is fine however their snap track switches are very limiting. I would use their custom line switches instead. They will mate with snap track. They are not powered which means you'll have to attach an operating lever or point motor to switch tracks.
I'm not a fan of DCC because I have too many older locomotives that would be tough to change to DCC and too expensive. For someone just starting out DCC provides the advantage of running more than one locomotive on common parts of your layout, without having to sectionalize the layout electronically.
It saves wiring, switches etc. Make sure your locomotives are DCC equipped or convertible. Couplers are a big question. Cheaper makes still use the horrible "standard" hook/horn model. Better ones use a magnetic coupler compatible with the brand named "Kadee".
You can fit "Kadee" types to most modern made cars and locomotives. They uncouple with a magnet inset in the tracks. The original "Kadee" couplers are better, but more expensive, than those that are copies. "Kadee" are metal most of the copies are largely of plastic. All need careful adjustment. Kadee makes a cool gauge for this.