Listen, this takes time, alot more time then people realize. All good things take time, so as much fun as it is thinking about everything that you can and might have on your layout, take your time. This is one of the best parts of building a layout, "the planning phase"
You are on the right track, sometimes if it gets too much, just step back for a bit, and take a short brake, in the time you take a brake, more good thoughts will come for you to make decisions.
On your curve radius. You have a bunch of room, more then most of us. Yeah 30 inch sounds alot, but with your space it really isnt at all. What type of rolling stock do you want to run?? Are you running passenger cars? The answers to these questions will help you figure out what min radius you should/can run.
With your space, dont see why you will have to go any smaller then a 26-28 radius, 24 is just too small unless its on a siding/branchline
I agree with Rico, i like the idea of a paper mill/power plant as im modeling a paper mill also, only wish i had the space you have
If you want to get fancy and have even more mainline run, you can pull of a "nolix" with the space you have running along the walls gaining elevation to a second level. Of course you can do this with a helix as well. The word "helix" scares some people