It depends upon your knowledge.
There are several programs that can be used to make scenery with GMAX is one but this program is not very easy to learn and  can cause severe hair loss trying to learn it . Gmax is the program a lot of game desginers use to make games with but it is helpfull if you have had some kind of experience with a program like this as it can drive you crazzy, and to pull your hair out.
Scenery builder is another program that from what I have read here on posts is a little more user friendly and you are less likey to loose any hair while using it.
As for mesh files there several sources out there that have tons of free mesh files all you have to do is just down load and install them. 8)