I'd highly suggest you take a look over at thedarkcity.net
There are several "good" builds with
DE depending on how you want to run them. You might consider just buying a few core units now then pick up the rest when you get the codex and have a look through.
By "core" units I mean start with getting 2 battleforces. This will net you 20 warriors, 20 wyches, 2 raiders and 6 jet bikes. Which is plenty to start putting together until the 20th.
In my collection I currently have:
18 Jetbikes
50 Warriors
20 Wyches
3 Ravagers
2 Talos
2 Razorwings
10 Incubi
10 Trueborn (all modeled with blasters)
5 Raiders
5 Venoms
Urien
Haemonculus
Archon
Lelith
5 Wracks
5 Scourges
5 Hellions (currently used as Beastmasters)
10 Khymera (conversion)
10 Razorwing Flocks (also conversion)
6 Harli's plus a Shadowseer
I haven't really counted up the points for that and certainly don't tell my wife what that cost.
My next purchases will be to expand my jetbikes to a total of 27.
Out of that list, the only thing I haven't fielded in the past year has been Lelith. Everything else has a place in my various scheming plans depending on how I feel like running them that day. My favorites are the Jetbikes and Talos: they just seem to regularly strike an inordinate amount of fear in my opponents when I put them on the table.
With this codex the main units to stay away from are the mandrakes and the archon's court (medusae, ur-ghul, etc). They simply don't do jack. Some people like Grotesques - but I haven't wanted to pay the price ($) necessary to field a decent sized unit of them.
Wyches are great tank hunters - but do little else.
Warriors are your back bone objective holders and you will win or lose a lot of games depending on how many you take and how you treat them.
Raiders are simply required but don't get too attached.
DE ride to battle, but walk home.
Reavers are spoilers with many many uses. Close Combat is NOT one of them.
Venoms are the ultimate in
MSU. Take care with how they are equipped (always dual
SC) and what is riding in them. Trueborn w/Blasters are good. Warriors with
DL are bad. Other units are situation dependent.
Ravagers used to be required. But it really depends on how you build your list because wyches fill the tank hunting role, sometimes more reliably. When I take wyches, I usually forgo Ravagers. That said, you will still want 3 of them for those days when you want to have a mobile long range
DL platform. Also, they have the same platform as the Raider so it's easy to proxy them as such -- just remove the side guns.
Talos/Cronos - These guys draw, and shrug off, an insane amount of fire. Which is a great way to keep your opponent from shooting at everything else. I watched one Talos shrug off 2 Vector strikes and baleflamers from 2 separate Helldrakes in a single turn; it then went on to wipe out a squad camping on an objective. Those attacks would easily have wiped a couple warrior units off the field. Next game I watched one live through 80 bolter shots then go on to crush the closest marine unit. In both games the firepower my opponents wasted cost them the game.
Razorwings - sometimes good, sometimes so so. It is an Infantry killer and should only be taken for that role.
Hellions - Some people say they are good in large quantities. Personally, I don't think they are all that so I use them as cheap Beastmaster conversions.
Kymerae/razorwing flock - Decent
CC unit that can cross the board quickly. I'm still playing around with how best to use them. They aren't like a pack of Flesh Hounds but they are good enough to slow death stars down.
Incubi - When taken in a unit of 8 or more, point them at a target and watch it go poof. Also do well as bodyguards with an Archon kitted out for close combat.
Haemonculus - Easy way to get a pain token at the beginning of the game; also turn wracks into troops. I'll often field a Haemi as my
HQ, with 5 wracks as bodyguard to camp on an objective in my DZ.
Lelith - Needs some wyches to be body guards. Has LOTS of attacks in
CC, but at S3... which really isn't that great... but they do ignore armor saves.
ymmv.
I'm not going to cover the other units as there are certainly some good guides online about
DE. I think my point is simply that before you pull the trigger you need to know what style of player you are/want to be and what your meta is like. Lots of tanks? Take units of 5 wyches in venoms. Lots of infantry: warriors in raiders plus a razorwing (or two..). Lots of fast attack? Keep in the raiders plus use a Talos or 2 to sit in the center for support. Lots of helldrakes? Pray your opponent is preoccupied with that Talos and keep out of their way.
The last four things I'll leave you with:
1.
DE function well when you have a solid plan for what each unit will do and how they support each other. Think layers. Some examples, Trueborn open up transports while the Venoms and nearby Warriors shoot up the contents. Warriors cause a
FMC to fall out of the sky (grounding tests) while venoms blast it. Talos draws heavy fire while your Reavers sneak across the board to bladevane units in the open. So combine overwhelming firepower with fast redeployment to make sure you are only fighting part of your opponents army with everything you have. Pick them apart.
2.
DE don't really have a middle of the pack attacks (ie: S6). They have poison and S8. Plan accordingly.
3.
DE function horribly when you are making things up on the fly or trying to play a list that simply doesn't gel with your approach. If things are going wrong, use your speed to regroup elsewhere on the board.
4. Expect to lose. A LOT. Until you figure out said approach. So be prepared that
DE are an army with a difficult learning curve. That said, stay away from most "upgrades"; especially the one shot ones. Those points are usually better spent on meat shields (aka more bodies).
Black Templars 4000 
Deathwatch 6000
