I'm my case I need many spray guns simply because I do custom painting for a living but for sure you really only need one gun for hobby work. if your only going to have one spray gun I would get one with a detachable cup makes color changing and cleaning easier.
if your buying automotive products for airbrushing your going to be spending more than if you buy modeling paints. the good thing is automotive products are really much better. sadly the smallest amount you will be able to buy is a QT in our case or a liter in your case. as long as you cap the can tight it will last.
search for U-pol 2252 primer. then lookup the TDS Technical Data Sheet it will tell you the difference mixes to mix it in 3 different ways. it is sold all over the world so my guess is you can buy it were you are.
you can also look up Transtar 6234 this is a sealer both the u-pol and Transtar are gray color.
2K does not include the reducer as part of the 2K. in this case it's the product and hardener.
I wish they offered more transparent sealers here in the states I always have to special order it.
wet on wet is just as it implies. you spray the sealer and let it flash off for about 15 to 20 minutes then you can spray your top coats and that would be either shingle stage or base and clear. but no you do not sand the sealer. you can seal over the bare plastic . primers are not always needed. in fact for most modeling I just hardly ever primer anything I just sand the plastic and seal it going wet on wet right to the top coats. the sealer will give you good adhesion and a uniform base to put your color over.
base coats are nothing more that the color you want what ever your painting to be. base coats will need to be clear coated.
single stage is another type of top coat this too is the color of what ever your paint you want it to be but you do not need to clear coat it. in most cases you use single stage with solid color that do not have metallics in them but they are also offered in metallic colors too.