Recently I wrote a review for undead in 3rd edition, you can find the links here:
As for your questions:
Undead have wonderful anvil-units. (Anvil = unit that can take on a beating without routing, preferably at a discount cost). In your collection, the zombie legion, revenants and skeletons furfil that role. In a 2,000 point army I would feature two anvil units, in 2,300 maybe three.
As for your collection, I would build your skeletons as two regiments (or a regiment and 2 troops) so you can either field them as a horde together or separately as regiments. Skeletons make great roadblocks and unlocks in regiment size, even better anvils as a horde. As troops, they are surprising resillent and they are amongst the cheapest units in KoW @ 55 points. Here’s an example of my skele horde:
Back in 2nd edition, I worte a “building on a budget” for the undead army which starts out with the Undead mega army which is somewhat like your collection. Even though it’s writen for the 2nd edition rules, some parts may still apply. you’ll find it here:
Edit: especially the analysis on unit role cq strength is off in the 3rd edition. Balefires were useless in 2nd, but quite valid in 3rd. /end edit
An army needs some of the following units:
-Hammers; i.e. stuff that kills.
-units that dictate the flow of battle. Anvils, chaff and blocking units fall into this category.
You have plenty of anvils. Combine the zombie legion and skele horde with a pair of ghoul troops and troop of wraiths, you’ll have enough tools to dictate the flow of battle. This means you’ll need some hammers. Amongst several options are:
c) Zombie trolls. (I love them!)
d) Revenant cavalry. (in hordes. As regiments/troops, they are not that effective as a hammer due to melee 4+ and only TC)
e) Soul reavers, both infantry and cavalry. @FredOslow hasposted a nice alternative if you hate the original models like me.
f) the hitty character option like vampire on dragon, revenant king on beast, vampire on pegasus.
There’s not much of a meta yet, but the battle reports I see favour soul reavers and revenant kings on undead flywyrm. I have a weakness for wights, zombie trolls and vampires on undead pegasi.
Other than anvils and hammers, you’ll need some support. Unless you go soul reaver/werewolf heavy, you’ll want at least 2 models which can both surge and inspire. Necro+inspiration talisman is usually my first, revenant king the 2nd, but there are other options. Lich king is loved by many and Mhorgoth the obligatory seems ahem exactly that these days.
Balefires have the nice advantage of putting pressure on the enemy by blasting their heavy hitters and/or engaging in counter battery fire. Though I haven’t used them yet (mainly because my other army -KoM- uses artillery extensively), I can immagine them being very effective if used in pairs or trios.
Hope this helps somewhat.