Welcome to the forum! It's great to see another ship enthusiast on board.
I'll try to answer your questions - but please remember that these are just one individual's reactions and opinions. I'm a fairly long-time (48 years) ship modeler who's worked in plastic and wood. I imagine your post will get some other responses, all of which deserve as much attention as mine does.
1. Different people find that different techniques work for them, but generally speaking airbrushing requires thinner paint than hand-brushing. I'm not familiar with the Revell airbrush paints you mentioned, but I suspect they don't need thinning for hand-brushing. I personally hand brush most of the time (though I do use an airbrush for some things). My personal preference is for acrylic paints; my favorite brand is PolyScale.
2. No primer is necessary to make enamel paint stick to plastic. Many modelers, however, like to apply a coat of grey (or some other neutral color) first, to make the final coat appear more even. If, for example, some of the parts are molded in black and others are molded in yellow, and the entire model is to be painted white, it will take several coats of white paint to conceal the underlying colors. An initial primer coat of grey would solve the problem.
3. The choice of a ship kit is entirely personal. Many modern modelers (including me) got their start in plastic kits (I built my first one in 1956 - I think), and find that medium more friendly than wood. Others prefer wood, which requires many different techniques. My personal opinion is that in the typical sailing ship model some parts can be made quite impressively of plastic, while wood has a clear advantage in others. Styrene plastic, for example, makes an excellent planking material because it's easy to bend. That same quality makes it a terrible material for small-diameter masts and yards - which are not supposed to bend.
4. The people who spend years on plastic models are replacing and adding to the kit contents. Some years back I spent several years on a model of H.M.S. Bounty, based on the Revell plastic kit. By the time I finished, it contained about a dozen of the original kit's plastic parts - and all of those had been heavily modified. When I finished it, I took a good hard look at what I'd done and decided to build my next model from scratch.
The most sophisticated plastic kits (e.g., Revell's U.S.S. Constitution and Heller's H.M.S. Victory) do take a long time to build - and they can be made into beautiful, historically accurate models. Plastic has the potential to be molded into finely-detailed shapes that no human hand, however skilled, can duplicate. (If you don't believe that, take a good look at an old-fashioned phonograph record.) The best plastic ship kits contain figureheads, stern carvings, and other ornamental castings that, to my knowledge, no wood kit on the market can match in terms of artistry and accuracy.
Unfortunately, there's also a tremendous variation in quality. Many of the plastic kits on the market are, in terms of historical accuracy, junk - and most of the wood kits on the market are worse. (We've taken up this topic several times in the forum. Regular participants undoubtedly are sick of reading what I think about it, so I won't repeat it here.)
The bottom line is that ship modeling, for most of us, is a hobby, to be pursued because we enjoy it. One great thing about it is variety - in terms of subject matter, materials, techniques, and personal approaches. My suggestion is to do it in whatever way you enjoy most - plastic or wood; two years per model or two weeks. If you're happy with the results, and you have fun, that's what's most important. If you learn something about maritime history and technology in the process, that's even better - but entirely optional.
Welcome aboard. It's a great hobby.