I will be shooting an indoor car show with my Century Graphic, Graftar 103mm lens. I will be shooting half hand held, half using a monopod with the available lighting. Which B&W film, or speed will give me low grain, sharpness , and great contrast?

if you are going to use roll film !! go with some thing like a ASA 400
I like Tmax ASA 100 but you may need flash
but 400 Tmax works better with no Flash as it two Stops faster
ASA 800 could be great but I don't know if its in 120 film

3 1/4" x 4 1/4" or 4" x 5" might fit better than 120 film, especially if you are using film holders or Grafmatics.