Since you're going to be doing inside shooting, you'll most definitely need to see about a decent external flash. As to which one you should, it depends on how much you're willing to spend.
I would say also that you need to consider getting a faster lens--a constant f2.8 would be good, which won't be very cheap. Depending on how much focal length you need, you've got a variety of options--
Short end--DA*16-50/2.8, Sigma 18-50/2.8, Tamron 17-50/2.8. The Tamron and Sigma are the most reasonably priced coming in at around $450 each.
Mid-tele--Tamron 28-75/2.8 or DA*50-135/2.8. If you don't need the extra reach of the 50-135, the Tamron is the best deal at around $400 new.
Long-tele--the soon-to-be-released 70-200/2.8s from Tamron and Sigma, priced at $699 and $799, respectively.
HTH,
Heather