You could get Oinkerwinkle Tools, just google "oinkerwinkle tools"
Those tools are for editing the existing blizzard models, like making them weaponless.
For a real modelling program, you could get GMAX a FREE modelling program, just google gmax.
I personally hate GMAX and I'm using MILKSHAPE 3D instead, it costs 25$, but it's worth it.
Well, the perfect modelling program is 3DSMAX, but it's VERY expensive (more then 200$ I guess).