I think external flash is a must. Get the one that can swivel. For balance, I think the FL36 is better. If u need more speed and power, then no choice, get the FL-50. But FL-50 I must warn u when mounted on the 510 is a bit unbalanced.
Also, the reason for getting the external flash is to actually AVOID the harsh look. By using BOUNCE cards!!!! This will give you very nice indoor photos. I am new to this, but IMHO, I think if you still do indoor flash photography without bounce cards, u are doing a dis-service. It makes the photos dramatically better. I am going to try it for myself ASAP.
Also, COMEX was selling the dual lens at drop dead prices. U should have gotten it then. Try Harvey Norman at Funan and see if they still do the dual lens.