Well, any place selling plaster should have different hardness grades on offer. Just get the hardest kind they have. Art & craft stores might have some resin plaster variants, that should be a lot sturdier.
If the major problem is chipped paint, I recommend pigments. If you just add a little bit of black pigment when mixing your plaster, the result will be dark grey, so any dents and scratches will be pretty much invisible.
Best of luck!