1. Hand painting is quite rewarding but also very slow. It is hard to get even surfaces like the airbrush but overall you can get OK finishes with practice. No matter what, if you are in for the long term, I think hand painting is a must, at least for painting details and weathering etc.

Airbrushing is very good investment IMO. Paint is a lot cheaper compared to Spray but almost the same as hand brushing since they use the same types. But the upkeep is a little high in terms of cost and effort, cause you need to buy washing solution and others. Airbrushing is recommended if you really want to paint, and love doing it. Personally, I have AB but I actually don't enjoy the process, so a bit wasted but I still feel like the investment was worth it.

2. This 1 I can't answer, don't keep up w brands and prices. But I do recommend getting the cheaper, electronic AB without compressor. I saw a lot of posts using them and they say it is easier to use. Also should get proper safety gear like respiratory mask and painting booth.

3. The main problem with using AB for long periods is the compressor overheating in my personal exp. Just buy better compressor? But even with my cheap compressor, before it overheats I usually finish my session. Cause there is a lot of waiting for paints to dry, and in between that you can clean the AB or prep some pieces. No need to rush through 3 colors in 1 session IMO.

4. Acrylics are safer from what I hear, while lacquer I hear is bad even with masks. Wear proper safety gear. As for brands, I am only familiar with lacquers, in which I like Gaia>Modo>Mr Hobby. I don't think any brands are "better for beginners". Just the chem components might be different, so you have to find what you like.

5. I still recommend priming. Washing with soapy water is to get rid of the residue on the kits, which has caused problems with paint sticking yes, but still need a good surface for paint to stick to. Alternative for priming is to sand your pieces so the surface is rough, which helps paint stick.

6. Yes, clean before priming. Personally I don't top coat before decals because I haven't had silvering issue, but some people have so you can experiment 1st.

7. Not very sure either, cause I did try to use the "thinner" from local hardware shops. Those cheap 1s like... Cap Kuda I think. The result is not very good. But also I tried at a time when I was a beginner.

8. If not mistaken, Tamiya paints should go for 10-12 for a standard short bottle. Some shops would sell them at 1-4 ringgit higher than that which is to be expected. Honestly like some said, it is harder and harder to get a good supplier for paints and supplies. For now I use Jshobby and Becky Customizer but both don't have huge variety of stock.
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Hope this helps. I'm not pro but just casual builder, so maybe someone here can help better.