Hi mate!
For mod i suggest you to use a couple of programs:
Pack3D, for unpack models from dat files;
3Ditor, for a lot of functions and settings, and for set objects coordinates (xyz);
(those 2 are avaiable from subsim, use search option)
after this you need a good 3d modelling program able to work on obj files.
There are a lot of 3d modelling programs, you can choose. I use Maya 7.
For poly, you can change numbers, but don't make files too large or you'll not be able to load them
If you need explanations, i'm here ^_^ regards, VD