Halloween II costume help

Hi, I'm new to costuming and I've ordered the trick or treat studios H2 mask and want to make an decent Halloween II costume (my first costume).

I've read that the Spruce Green Red Kap Twill Action Back coverall is the way to go. I was wondering if anyone out there could give me some tips and tricks on how I would weather the coverall. Should I just wear it and fix a couple of cars, or is there a better way without smelling like an mechanic?

Also, can anyone point me to the direction where I could obtain a decent prop knife?

Thanks.