Hi.
to the FSM forum.
As for your questions:
I won't speak for aircraft, but for armor, I'm sure you'll find the general consensus goes to Tamiya being the best. And generally, their parts fit is the best. But let this not pry you away from other kit manufacturers, as they all have their ups and downs. For armor, at least in the most common subjects, such as the Bradley, Trumpeter quality is crappy according to most, but I find their price to be the best, and I've built quite a few of their kits. True, they need a little more work, but the end result is just as good.
Aftermarket parts are a good way to enhance the look of the model, but be prepared to pay the price. Also, there are aftermarket conversion sets for existing models to make vehicles not otherwise made by kit manufacturers.
Many will attest to the greatness of AM (after market) but I'm not much of a fan. They can raise the overall price of a kit substantially, and usually don't offer enough in the way of super details. I think you'll find that a kit built out of the box, if done well and with care, can look just as good, if not better, than one fully laden with AM parts. Plus, many offer details that you can make yourself out of styrene sheet, wire, putty, and aluminum foil for much less.
As for Group Builds, they are exactly as they sound. It's a bunch of people getting together to build one certain subject, be it a particular vehicle, or a particular group of vehicles, such as the British and Commonwealth GB now in progress.
It's a great way to share your experiences, learn as you go along, and see others do the same, in a friendly comaraderia. Highly suggested