Aside from the End Times (of which there is probably only one more volume to come),
GW only really release new units with each new army book, which means that, for one particular army, there aren't any new releases for several years after their book has hit. However,
GW's recent move with the new Tyranid models is a change from this demographic but it's more likely something to patch up the holes left by the Chapterhouse shenanigans, so I'm not convinced it'll spread to other armies or to
WHFB. And, of course, you don't
have to buy the new units. Sure, some might be good but none of the End Times units released, for example, have been auto-includes (at least not with Nagash, I haven't read the Chaos one). You can easily get by without them.
As for uniqueness, it is a lot harder than in
40k as all your models (pretty much) are in blocks, making a single, converted model much harder to see. So, as Shas says, the best way to add uniqueness is through unit fillers. These things are great, as they add flavour, variety and save you some money in the process. There's still plenty of opportunity for conversion or cool basing, but you have to save them for models that aren't in a unit, or really small units (like 5 man cavalry units).
For unit size, bigger is better as far as infantry go though, as you point out, you can go too far. Most non-elite, combat infantry units should be fielded at least 30 strong. This is usually in a horde formation, which allows all thirty models to attack. Then, depending on the cost of the unit, it is common practice to add another rank on the back to soak up casualties, but more than that is rarely necessary. In most cases, ranged units shouldn't be fielded in units more than 16 or so since this starts to limit the number of shots (HE and
WE are common exceptions though). Cavalry should be fielded in cheap units of 5, or bus units of around 15, nothing inbetween. Monstrous Infantry work anywhere between 3-9, or more if you're Ogres and Monstrous Cavalry tends to work best between 2-4. Really though, the best way to know when you've gone too far is points value. In a normal sized game (2.4k) it's usual to see units costing around 500pts, but anything more is usually a bad idea. You want to have plenty of room for other units, characters and so on. Speaking of characters though, it's worth considering where you want to put your characters. You shouldn't really keep all your eggs in one basket, so to speak, since there are a number of spells and effects in the game that can do huge damage to such units (and you don't want it to run off the board after a bad
Ld roll either).
You seem quite interesting in playing Wood Elves but, sadly, out o the four you've listed, they're probably the worst beginner army. Don't let that put you off if they genuinely are your favourite, but they do tend to require more finesse and skill than other armies and they tend to focus more on shooting, which is not the norm.
WHFB games are usually won in combat (and movement, to get to the combat), so it's worth getting to grips with that. High Elves would probably be the best because, whilst squishy, they're a good all-round, strong, forgiving army that will able you to learn most aspects of the game. Dwarves are similar, but their lack of magic and unique rune system leaves some gaps.
WoC are another strong army, since they're simple, effective and have a low monetary cost, but they stop you practicing the shooting phase. Ultimately though, they're all good armies, so if one is a standout favourite, go ahead and start!
I hope this helps, if you have any more questions, feel free to ask!
