You should really check out Reece's batreps;
http://www.dakkadakka.com/dakkaforum/recentTopics/showTopicsByUser/0/2776/17.page
He plays some really good footdar lists.
You are going to need to get a few extra models here and there just to bump the units up to minimum size. Right now what you have for full units are;
Farseer, unknown gear - is this a spear? If it is it is a singing spear. This is a great addition to any list.
Howling Banshee super exarch thing - probably Jain Zar, good news is she is a cool model with interesting rules...bad news is she is really far from competitive
5x Striking Scorpion + Exarch - Does the exarch have a claw? If he does this will be a great harassment unit. If he doesn't they will be an okay harassment unit just lacking the oomph.
4x Dark Reaper - This is a great unit with star shot missiles. You can make this unit top level competitive with a Tau buff commander. I would get an exarch to for the unit and you will need something to either bunker them or tank shots for them to make them top level. Don't forget to use guide and doom to unleash this unit.
5x Dire Avenger + Exarch - If you are running foot you will need to up this unit size to 10 to see any positive effects.
15+ Guardians and Weapons Platform - This is a good unit but you will need to decide soon if you are goining to be Biel Tan or Ulthwe Either way these guys can do good work with good play. Don't get trapped into giving them a bright lance and shooting pot shots at the opponent. Guardians need to get into shucat range and annihilate something.
These models will not make a minimum unit;
4x Rangers (+1 to minimum, very decent scoring unit)
2x Harlequins (+2 harlies and +1 shadowseer for a semi decent unit, not very good in current dex)
I actually like 2x brightlance, 2x flamer wraithlords. They are not the most competitive units but they can be used to good effect.
Dark eldar can be done really well with a minimum warriors and beast pack, baron + helions, or ravager + warriors. These all work well but make sure you get something from the rest of the choices.
DE don't really add that many things that eldar cannot do so make sure you find one of those.
I would be okay with jetbike dino riders. Slap some wings on them and they would be even better.
In an ulthwe list the avatar is almost mandatory. In most footdar builds the avatar can find a place but again he going to be a tough choice in a tournament. There is a fun elfzilla list with the avatar and 3x wraith-lord/knights.