Welcome to the forum VooDoo! Glad to see that you've come around to Fantasy. I wouldn't say that 'End Times' has really made the game any more mature than it already was - if anything, it's making the game more similar to 40k (allowing allied lists), and that's something that most veteran Fantasy players consider a bad thing, seeing as they feel WHFB is already the more mature of the two games.
As an aside, it's worth pointing out that while GW will force people to play against you even if you're using the 'End Times' supplements, most tournaments outside of GW are imposing bans on the added rules. Likewise, any pick-up games that you play outside of GW, and within some groups, will be a toss-up on whether or not the rules are allowed. Fantasy as a core system is considered to be decently balanced, and some of the things available from the 'End Times' will really botch that up. So I wouldn't base your decision to make the switch entirely on 'End Times', unless it's from a fluff perspective.
For the meat of your post though, you wanted to talk about armies. Going through your list of options, and what you want, I can give you this overview;
Warriors
Offer: deadly characters, average magic, chaos gods, elegance (Slaanesh), a reputation for cheese, featured in End Times
Pros: a very solid book/army overall, few truly awful units, low model count (cheaper, faster to paint, etc)
Cons: they're done to death already - people are learning how to beat them
If you want to go with Warriors, you can't really go wrong. They bring heavy armor, and can play in every phase except shooting (although their magic is just average). They can take a beating, and you can have anything from a "pushbroom" army of infantry just focused on getting into combat, to a horde of trolls, a mobile cavalry army, or an "air force" or "monster mash" army. They are generally a very fast army, and their characters are some of the deadliest in the game.
Skaven
Offer: somewhat random, decent magic
Pros: they truly can do everything - shooting, magic, and combat (I'd say movement is their weakest phase). Another army known for being solid all around.
Cons: lots and lots of models, prepare to pull off gigantic numbers of casualties
Skaven are the horde army of WHFB. Compare them to 40k Tyranids, if 'Nids were actually one of the better armies in the game. You're going to paint up huge units of rats, and then pull off dozens of casualties at a time. You win through the grind, not because your characters or any of your units are "super killy" on their own. They're not really that random, they just have lots of warmachines. Orcs and Goblins are the most random army in the game, if that's what you're really going for.
High Elves
Offer: really pretty models, armor, decent characters, excellent magic, one of the cheesiest items in the game
Pros: another small army, tactical playstyle, another very solid army, compete in all phases (shooting weakest),
Cons: some really dodgy units, "Elf" fragile (still the toughest elves overall)
The High Elves are a great army, but there are a few 'trap' units in their book that you just won't ever really play in a competitive list. They have a lot of units which fill the same role, and have very similar (or identical) point costs, so 2/3s of the units are usually hotly debated messes. Get past that, and the army itself is very solid though, and performs well. They can actually bring lots of armor to the table, with their Knights, but are still Elves with T3 - so it's the difference between Eldar and Dark Eldar really.
Vampires/Undead
Offer: killy characters, lots of magic, featured in End Times
Pros: can be a solid army, entirely immune to Psychology
Cons: another horde army (even more pricey than Skaven), very little shooting phase
Vampires are a good army if they're built right and played correctly. Otherwise they're a stalwart army, but they're ultimately doomed. They are driven by their characters - they become very slow if there are no Vampires around the units, and they rely on their elites to win combats. Like Skaven, expect to buy and paint lots of models, just to pull them right back off the table. They are fun though - showing up with lots of Gheists can be nasty, and the Vampires themselves are on par with Chaos for deadly characters.
-- When it comes to the history of each army, it's a personal choice. I really like the High Elves because of the huge hand that they played in shaping the Warhammer world as we know it. The Chaos fluff is great for being dark and twisted, and the Skaven fluff is just insanity. VC fluff is Dracula, ultimately.
For using Nagash, you won't unless you're using End Times. His huge points cost means that you can't field him in normal games until you hit 4,000pts, so you won't see him. At 2.5k he's going to be half of your army, and he's not impossible to bring down. It's like throwing a Baneblade on the field in 40k - big investment, so he needs to take half of the enemy army with him, and usually can't.
1k is just about the smallest you can play before the game starts to get a bit wonky. Because you play on a percentage system instead of an organization chart, and because of the wide array of upgrade options available to each unit and character, plus the rules' emphasis on larger units rather than small ones, the game just balances more nicely at 1k+. 2.5k is the regular game size, along with 2400pts (to keep the math simple).
Of all the armies you listed, WoC and Skaven do the best in small games. Skaven because they have cheap characters and units, and can therefore bring an army that feels more like a full, rounded force. The WoC do well because they are an army defined very much by their Core, so even if you can't afford to pack in the elites, your troops are still on par with virtually every other army's most elite soldiers.
If you've played Eldar, then the High Elves won't feel out of place to you. Likewise, Grey Knights are/were known for being dead-hard and bordering upon "insta win", so you'll get the same feeling from playing Chaos. I'll caution you that Undead are nothing like Necrons, because Necrons are "MEQ" in 40k, and Undead in Fantasy are anything but resilient or elite (except for their elites, of course).
I'm leaning very heavily towards HE and WoC for you, but at the end of the day, it's your choice.
Skavens and VC might be worth a look though
