Brokkr Hi and welcome

Q's 1: The limit for a direct x model is less than 10000 polys. More than that and your level will probably crash, maybe not

A good number is around 3000-5000, I tend to stay under that so characters perform well.
Q's 2: ScooT9 has answer that one. I have used a terrain generator called Earth Sculptor with great results.
Has an unregistered version but comes with limited texture size of 512x512. A registered version costs $50 now but well worth
it if you intend to use a lot

with the texture size 4096x4096
http://www.earthsculptor.com/index.htm
Q's 3: If your good at 3d modeling then Yes, one can make a hut/structure that the player can enter. Static of course except for any doors...etc
I have found the easiest way is to build it in sections from the ground up with the roof section last. That way if you want to add entities inside
such as tables,chairs,furniture..etc makes it much easy to place correctly.
My games never have bugs.... they just Explosys's
* I lost my Big Yellow Button. Do you know where it is?