[Outwest34au]

I would forget about the 3/16" unless specifically required. If I was going to bolt through something with a nut or just tap a thread I would go the metric. Having a mix of 4mm and 3/16 scattered through a car could make for some crossed threads down the track.
Also, beware the 9/16". It is a size not overly commonly used and the cost of 9/16 bolts can sometimes be double what you pay for common step sizes like 1/2 and 5/8.
Also need to decide what pitch is required for the metrics. With pitches of 1, 1.25 and 1.5 being common among different components (and the odd 1.75) you still may not have the bolt you need to bolt into that x(insert random component here)x thingy.

Your money, your project, your call.
Only you know what you will really need.