Since my attempt to discuss modeling the German Railways before 1945 has not been a good one, let me try post 1945.
Do I model the West German or East German system? Maybe I should jump to 1990 and a reunified Germany?
I do not know which one to suggest. Being born in 1957, I would definitely go with late Era 3 or early Era IV when diesels were red, first class cars blue, diners and sleeper red and the rest green or silver. Just the TEE trains added some color. I would go with the west, more equipment, and a nice mix of older and newer engines.
A lot will depend on what kind of trains you want to run -- and what kind of scenery you like. Do you want mountains (Alps), or coastal harbors or rivers?
West Germany, with its larger industrial base, certainly offers more possibilities -- and there are more models of western railroad equipment available. On the other hand, I noticed that Roco has quite a few eastern models in its current and upcoming offerings.
Both an electrified double-track main line and a single-track branch line (steam or diesel) have their appeal. And, of course, the amount of space you have available will be a factor, too.
I would not want a layout without at least one large industry -- large enough that there are multiple switches within the industry, so that you can do actual switching within the industry. (In many cases, these industries will have their own internal switching locomotive. The main railroad may either deliver and pick up at the industry -- or if the industry switcher is operated by qualified personnel, it may operate to the nearest mainline station to pick up and set out cars.
I hope I'm not sounding like a broken record, but there are a couple of fairly lengthy Blog articles on the considerations that can go into layout design here:
Take a look and let me know if you think those points are useful.
Since I have combined US and Euro trains, with an emphasis on military trains, I have chosen post ww2 at a us military proving grounds (ie Aberdeen in Md) so I can do both. Ft Eustis , VA got a lot of German ww2 equipment to test during and after the war. I have visited that facility and got many ideas from it.