Not sure if you still check this, Mikee, but I am Mario from Xonotic, and have some information on the IQM vehicle model problem.

The vehicles are invisible in newer engines (and lag alot) and are unable to be exported as .dpm, because of vertex groups. Here is a simple Blender guide to fix it from a friend:

(object mode) select mesh, select armature, Ctrl+P > With automatic weights.

Shift + right click to select multiple meshes in 3D view.

I think the models should also be .dpm, as .iqm even when fixed seemed to lag really badly to me.

Hope you're able to do this, as it would be nice to have better performance with the vehicles.