Not that I am an expert on Dark Eldar or anything because I have been playing for years with Chaos space marines, but I have recently switched to haemonculus covens. From what I understand though Wych's are pretty useless tbh they aren't even good for overwatch/cannon fodder. While I like the Haemonculus as well, They really need to be taken out the haemonculus covens supplement codex.
Now if you have a look at the Realspace Raids part of the commorragh tactica section, look for The Repugnant Ramblers by Mushkilla. I think one of Mushkilla's battle reports is a couple of pages in and this will give you a good grounding in what you need for a Haemonculcus covens list.
Usually replies breed replies though so someone with the experience you are looking for should chime in at some point. In the mean time while you wait have a look at everything on the forum tactics/discussion/army lists. I already have and it has helped tremendously
, This is by far the best forum in all of 40k no joke.
Now I don't have the experience to tell you for certain what to buy/do, So if you can have a look at grotesque's, Talos, Cronos, Wracks, Mandrakes from the Heamonculus covens book that will help a lot. But from what I understand Wracks and Mandrakes are not the best choice out of the covens book.(some people like them some people don't it is really up to you)
Now dark Eldar are a fragile highly mobile, long distance dark light shooting army. While the Haemonculus covens are close combat based with high toughness so they can take a licking and keep on ticking. Hopefully this has helped you a bit, also post a hello on the port of lost souls this is always a good start for new people to the forum.