I can't speak with much authority, but I can say that with your detail specs and as long as you're rendering with a smaller resolution (under HD) you shouldn't have much of an issue. Terragen, with a little bit of work, can achieve photorealism without a big hit on performance. Even I can do it, and I'm one of the newest members here.
You're biggest render time sinks are going to be in hi definition clouds and water. There's a lot of optimising options you can change to get the fastest renders possible.
I haven't done it, but I know Terragen can use real world geographic data, so you're fine there.
Terrain also doesn't take much to render compared to the rest of the scene, and with a bit of work, you can get some really spectacular shots.
For this project, I'd feel safe saying Terragen will save you time, you'll just have to keep in mind that everything's generated procedurally, no need for textures and materials and all those lovely settings. Instead, just plug a few nodes together, fiddle with the sliders and hit render. Terragen does the rest.
Although, you can fiddle with all those lovely materials if you want
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