It's probably me, but I think you're making this a bit too complicated.
Since you won't need to animate it and only need stills you can get away with a lot of cheats.
For instance: you won't need to animate a "from orbit to planet surface" shot, so there's no need to create a planet with continents *which is also* detailed enough at very low altitude/planet surface. No one will ever be able to tell "hmmm that mountain in the background looks sooo different from space"

You could even create a separate .tgd/project-file for every type of terrain you're trying to achieve. Getting everything on one single planet is just unnecessarily complicated as you would need to mask out everything separately.
Search the forum for "continents" etc. and you'll find some projectfiles of entire planets with continents and seas. I believe "cyphyr" did a fair amount of work on this. So you can use that for space shots.
Depending of what you're after I'd then move to DEM-files for pieces of realistic terrain base for low altitude/close up shots.
We can all help you with that, just don't try too much at once.
Cheers,
Martin