Absolutely-  Revenants were added late  (WD #186 )
but are still very much a part of (and legal in) 2nd ed-
The problem with Revenants is they are hard to find,
especially at a reasonable price (Still looking to add four, myself)!!
I would say the 'must have' unit is a pair of Wave Serpents,
Wave Serpents carrying a unit of either:
Exarchs, Banshees, Harlequins or Scorpions (IMO: Harlequins are Best).
It's a solid eldar tactic, perhaps the strongest one they have
(*note that Wave Serpents design change between 2nd eds 'Floating Iron'
http://images.dakkadakka.com/gallery....ent.jpgand 3rd & 4th ed's vaguely Heart-shaped version
http://www.craftworldgames.co.uk/p....C+ELDARis quite radical; make a conscious choice as to which you want! )
Next; definitely the Revenant squad, or
a Tempest Heavy Tank, eventually leading to a Company-
Actually: these are as good as Wave Serpents;
whichever one you find first i suppose-
I'd also seriously consider adding  some squads of Aspect Warriors;
One or More of the aforementioned Close Combat experts in your Serpents,
but also Dark Reapers for the punch; Spiders if you  can find 'em.
Less needed would be the Dragons, Avengers, or Hawks
These are not as important in larger games,
but in smaller games are suprisingly useful-
And for an Eldar Army, a flavour essential. Likewise: Get an Avatar!!!
Cheap to purchase, Free points cost, & Fun (especially in smallish games)--
A Doomweaver unit is good fun, but can be added later; much later.
The prism cannon is almost an after thought (except in -very- small games)-
Ditto the Warp Hunter; they can wait-
(although the WH is great against Orcs & IG, it's very, very Random)
A -lot- depends on the size of game you are  playing;
in a 500 point game, a Prism Cannon is more cost effective than a Tempest
(although neither are as good as a Wave Serpent or a Revenant)
Also important to consider: Availability and Price (goes without saying, I know).
In the US; ChaosOrc is an okay place to find stuff (at about High eBay price):
http://www.chaosorc.com/index.php?cPath=83_40_63In the UK; CraftWorld Games has quite good service & decent selection:
http://www.craftworldgames.co.uk/p/cate ... 449-Eldar/If you are only adding specific single bitz & units,
its probably worth getting them from such a re-seller;
EBay generally works better if you are expanding your army significantly at one go;
I'd say about £50.00 GBP /$80.00 USD is about the efficient cross-over point.
One more random point, Just FYI:
NetEpic is essentially an 'edited' version of 2nd edition;
the points costs were slightly adjusted & standardized,
and a few rules tweaks were added to enhance playability.
It is worth looking at eventually, if/as your games get bigger-
