You asked for opinions..... first, a 10 x12 x9 space is pretty tight for a car.....in all three dimensions. I will be similar to painting in an elevator and the hose is going to be in the way or kinked.

Second, in a tight space, the box fan will be blowing directly on something and the exhaust fan will be pulling air faster at one end of the car than the other....I think a balanced flow is better....a bigger room helps the flow. I personally feel too restricted when painting in a two-car garage. I painted Imron last weekend in a room with a 14 ft ceiling, two pusher fans that were 25 ft away and a 48" exhaust fan. With a respirator, the fumes were still something I could smell although not bad.

A home air filter can be decent or terrible....the filters with the punched round holes (bottle cap web scrap) and fiberglass that you can see thru will pass stuff like sanding dust. Get a filter rated at least MERV 5 or so...higher the number, the smaller particles it will trap. Use the filter that you cannot see thru...like the white 3M jobs.

If you put it on a box fan air supply, you have to tape it all around. Then, most cheapie box fans do not have enough head pressure to overcome more than perhaps 0.1 inches of water resistance...not much of an air supply. The box fan is probably only a small portion of the supply if the outgoing fan is quite a bit larger.

Regarding primer...I personally think that every coat should be as clean as possible...even if you sand it.

Make sure you really wet everything down to hold the dust.

Test your plastic building before you go too far to see how much dust it attracts due to static buildup......if you have a ceiling with a lot of dust stuck to it, some of it is going to fall during the paint job.....

Good luck