(Why do I suddenly have the image of Rowan Atkinson blissfully operating a layout in my head?) How big is your spare bedroom? Are we talking 3 x 2 meters? Bigger? Smaller? The size of your space and alotted layout area will determine how much layout you can squeeze into a given area.
My layout room (a shared space in the basement) is about 11 feet by 13 feet (3.3m x 3.9m*) which would allow me to build a fairly decent model railroad layout in there. However, since I'm currenly living with my parents, my available space is 2.5 feet x 12 feet (75cm x 360cm), which limits me to a small HO scale switching (shunting) layout. I could do more in N, but I have bad eyesight (I wear trifocals), so HO is as small as I can go.
Now, you mentioned a few features you'd like to have on your layout, like mountains and varying track height, and that's fairly easy to do on a layout, regardless of size. Three trains running at once is a tad more difficult in HO, unless you a.) ran shorter trains or b.) built a double-decked layout. N guage will allow you to do more with less space.
That's just my input, I could give you more advice if I knew more about what you'd like to do. Do you plan to model railroads here in the U.S., Austrailia, Japan, Germany, U.K. ...? Are you going to have small manifest freights or long coal or iron ore drags? Do you plan on having any passenger trains? Just currious.
Well, keep us up on the progress of your layout and hopefully you can acomplish what you're setting out to do. And don't be affraid to ask questions, there are a few of us who have "been there, done that" and are willing to give our advice on whatever problems you might be facing.
* I think my math is right. f (f=feet)x 12 (12 inches to the foot) x 2.5 (2.5 cm per inch), then move the decmil over two places (as there are 100 cm per meter).