First, I would recommend using the CVS version of Crystal Space rather than the 0.98 release, which is quite old by this time. There are many, many improvements and bug fixes in the CVS version. You can find daily snapshots at http://www.crystalspace3d.org/cvs-snapshots/
or just access CVS directly.
1)Ok, now I've used Irrlict and Ogre3D up to this point, so could you please give me some starting points, and some references to tutorials?
The simple tutorials in the CS manual (presently section 4.3) will be a good starting place. Once you've read that, the rest of chapter 4 can also be quite useful.http://www.crystalspace3d.org/docs/online/manual/
Another good place for tutorials is the community site:http://community.crystalspace3d.org/
2)Can I remove some of CS's API (network)- because our game is single-player- whitout crashing the engine?
Crystal Space features a highly modular plugin system. You load only the plugins which you need for your game and are free to ignore the rest. Almost all features of Crystal Space are provided by plugin modules, so you pick and choose exactly what you want to use. (Also note that networking support has been removed from the CVS version of CS. You can find some higher level networking support instead in the CEL project.)
3)Does CS have terrain rendering (LOD) available? Can there be more terrain meshes (3) at the same time?
Yes it does feature such a terrain plugin. Use the CVS version of CS for best results.
BTW, Personally I think CrystalSpace is HUGE and in 29mb, you could at least provide some precompiled dll's or even a SDK
Unfortunately, this is rather difficult because of the plethora of platforms and compilers which CS supports. Unlike projects with plain C interfaces, CS is written in C++, and there is no agreement or compatibility about object layout and mangled names between compilers and compiler versions. For this reason, we would have to offer a very large number of pre-compiled SDK's in order to support all of the platforms and compilers and compiler versions with which CS can be used.