I remember having a discussion with a high-level executive from a very large tool brand. He was describing to me how fillers are being added to PP injection molded products. I believe the ‘smoked’ effect you’ve indicated is from one of these fillers, more precisely, talc. You get the added benefit of a less costly material, lighter weight product, no loss of strength/integrity and also the visual effect.
If you you know PP is OK,before you make a injection mold ,you need’t know the accuracy .Because the pp shrink rate is same,after you have a mold ,you can provide different material to inject,then you can test these sample to decide which kind of pp to use .
For a tool box, I wouldn’t worry too much about flexural modulus, unless you have molded-in snaps. If you do have molded-in snaps, then that solves your problem, as the flexural modulus will be a design variable, along with wall thickness in your snaps. Check a PP design guide for these formulas.
You also shouldn’t need to worry about flow rate. If the PP you’ve picked is provided in pellets, it’s suitable for injection molding. Your toolmaker will handle the exact flow design.
Pick the highest impact resistance you can afford. I’d invest in this though, it’ll affect your overall quality drastically and will cost you more in the long run if you cheap out on your plastic.