An Archon in with some Warriors. Ravager, Razorwing, Scourges, and/or Reavers riding for anti-armor. A Heroes Path formation, and three Fast Attack Venoms for them to ride in. That seems like a solid core. I've run that list before (and variants thereof) and done quite well with it.
Here's some ideas for running DE and Harlequins together. That's how I do it, at any rate.
As to future purchases, I think you'd do very well with some more venoms. Five man warrior teams in a Venom (one blaster per team) is a solid way to field your troops.