Well, I'll throw in my two cents' worth.
The number of parts in a kit varies tremendously with the scale, the manufacturer, the material (styrene vs. resin), and the age of the kit. Some of the oldest 1/700 battleships and carriers on the market (the old Revell ones from the 1970s, for instance) have fewer than a hundred parts apiece. The more recent 1/700 battleships and carriers from companies like Dragon and Trumpeter generally have between 200 and 300. (A carrier will generally have more, because of the airplanes.) Plastic kits in 1/350, generally speaking, have more parts. I believe the Trumpeter Nimitz-class carriers, for instance, have upwards of 1000 parts apiece. All other things being equal, resin kits generally have fewer pieces (because resin can be cast in flexible rubber molds, where as plastic has to be injection-molded).
I happen to like 1/700 scale, for a few reasons. The variety of kits available is greater (though 1/350 is catching up), the finished models don't take up much space, and they're relatively cheap. (I just can't spend several hundred dollars on a kit.) There are only a handful of 1/500 kits out there. Heller used to make a rather extensive line in 1/400, but they're harder to find and the subject matter is limited. (Heller made French and German ships, and a very few British ones. That's it.) A couple of manufacturers have dabbled briefly in 1/400, but beyond Heller's there are only a handful of kits.
Generally speaking, I recommend that newcomers start with a large-scale kit of a relatively small vessel. Three that come to mind immediately are the Tamiya 1/350 U.S.S. Fletcher (destroyer) the Trumpeter 1/350 U.S.S. England (destroyer escort), and the Revell 1/350 Type VII U-boat. All those are reasonably accurate kits that can be built into impressive replicas in a week or two. And they don't cost much (relatively speaking). I recommend cutting your teeth on a small vessel before shelling out a staggering amount of money for a 1/350 battleship or carrier, which will involve mind-numbing repetition (forty or fifty 20mm guns and thirty or forty aircraft), and take months to build.
You can figure on every kit having either plastic or resin parts. Beyond that there's a great deal of variation. An increasing number these days come with sheets of photo-etched brass detail parts - but a lot don't. Some resin kits have a few white metal cast parts, and a few plastic kits have resin details included. There is an extremely extensive aftermarket, which sells brass, resin, and cast metal fittings for all sorts of ships in all sorts of scales.
In many cases, a kit comes with a small sheet of etched brass parts that are specific to that ship, but not such generic items as railings and ladders. For them you have to go to the aftermarket.
A lot of kits come with decals (for such things as hull numbers, aircraft insignia, and carrier deck markings) and flags - either in decal form or paper. Some don't.
Recommending manufacturers is fraught with danger. I can think of at least a dozen companies each of which has made at least one excellent kit over the years and at least one that, by modern standards, is pretty crude. (Most obvious example: Revell, whose recent Bismarck kits have a fine reputation but who's also selling an Iowa-class battleship that's been on the market since 1953.) Off the top of my head I can't think of a Dragon kit that isn't recommendable. (Dragon has only been in the ship market for a few years, and almost all of its kits are pretty close to state-of-the-art.) Your best course in choosing a kit is to look up reviews of it on the web - or ask here in the FSM Forum if anybody's familiar with the kit.
Hope that helps a little. Good luck. It's a great hobby. And this Forum is a fine place to get support and encouragement. There are some decidedly odd people here, but most of us are harmless.