Battalion box should do you well to start, reavers are a good unit to have as well!
I'd definitely recommend ebay and kijiji, that's where I got 70% of my army at less than half retail. If you're patient and pay attention you can snag some sweet deals.
Check out this site, dedicated to Dark Eldar in every which way;
http://www.thedarkcity.net
I'd recommend grabbing a venom and an extra raider or two, and you should have a fantastic start point!
Best of luck, dude