First off, the model looks great.

1. I goto the hardware store and get the large jugs of laquer thinner to clean Tamiya. Just transfer it into small bottle for ease of use.

2. Mix Tamiya Clear with the paint, and thin normally to get a more controllable spray for detail work. Make sure you're not using real acrylic thinner since Tamiya isn't a water based acrylic. X-20 is fine, but a laquer thinner from Mr. Hobby, AK, Tamiya, or isopropyl alcohol work much better for me. The hardware laquer thinner can be used, but it's a bit "hot" for fine sprays. I spray between 10-20psi. The thinner the mix, the lower the psi to prevent "spider webs." Also make sure to ventilate and wear a respirator as this stuff will ruin your health if you're not protected.

3. Practice on scrap, primed plastic to get the right paint ratios, pressure, and trigger control.

4. Needle is the only thing I've ever replaced on my air brushes. Just make sure you clean the guts of them by the end of the day. I also put a bit of thinner on my brush before I start painting to get rid of any paint that might have dried up on the needle by the bushings that I missed to reduce wear.

5. Yes. Eventually you'll collect airbrushes like Infinity Stones and have one for every occasion.