Welcome back to the hobby!
1. Citadel paints are water based acrylic. Just about every paint line aimed at miniature painting will be water based acrylics.
2. There are a glut of brands on the market at the moment, with some being more or less readily accessible depending on where you live. The most well known one would be
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Vallejo. They have hundreds of paints across multiple ranges (Model Color, Game Color, Air, etc.), so you can nearly always find something to suit what you're looking for. They're also widely available, so you shouldn't have much trouble finding somewhere that sells them.
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The Army Painter. Another commonly used brand with a rather large selection of paints there widely available. The Fanatic range is proving to be quite popular.
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AK Interactive. Their 3rd
Gen range is very highly rated.
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Scale 75. An interesting brand in that they use a gel based medium for their paints rather then water based. Still perfectly acceptable for model painting and can be mixed with waterbased paints with no issue. Absolutely astounding range of metallics, among other things.
Those are probably the most common and accessible paint line used for miniature painting. Others that might be harder to source include.
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Two Thin Coats
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Monument Hobbies: Pro Acryl
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Ammo by MIG (Not paints as such, but does a wide range of technical "paints" used to enhance paintjobs.)
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Turbo Dork
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Kimera Colors
There's no
One is Better Then The Rest paint line when it comes to miniature painting. Some people prefer a particular brand over others, for what ever reason. While many people mix and match paint brands as they see fit. Even Citadel, expensive as they are, shouldn't be overlooked. They have some absolutely amazing colours.
3. Most brands will do paint sets. Vallejo have a few different types, as do Scale75 and Army Painter. Searching a brands website is often the best place to start looking.
4. I've just recently got myself a new airbrush/compressor and it's been quite useful for priming and basecoating. I don't use it for anything more advanced then that though, so my opinion on airbrushing is based around that limited set of uses. Most people will start throwing brand and model names at you with claims of being "cheap" and beginner friendly. But it still ends up being a big investment for something you might not be sure about how useful it will be.
I bought a offbrand airbrush/compressor combo off ebay for $140AUD and honestly I think that's all you need for doing basic work like priming/basecoating. Its certainly a the cheapest way to dip your toes into airbrushing. Considering even branded beginner airbrushes can cost hundreds of dollars and that's without a compressor. If you find airbrushing is for you, then great, you can safely start looking at more expressive branded brushes. If not, well you haven't broken the bank on something that you might find is hard to sell on.
I'm sure others will... vehemently disagree with me though.
5. I don't think it makes any difference what sort of superglue use for plastic models. Gel might be easier to control, but I can't imagine either type will do a better/worse job then other.
Hope some of that helps you with what you were looking for.