multiple skins, single texture, single surface. if you want multiple surfaces/textures you'll need md3 or iqm.
player is 56 units high (bbox of -16 -16 -24 to '16 16 32', firing position at +16, view/eye position at +22).
up = +z. forward = +x. doesn't really mean that blender will display those same axis (exporters will typically translate stuff like that).
I don't export to mdl, although I'm somewhat tempted to update my iqm exporter to spit out mdl+md3 files instead. iirc taniwha made an mdl exporter for blender but I've no experience of it.