I think I'll settle with announced white dragon and NPCs for now. I was thinking of introducing rival party of NPCs anyway, so my PCs have some competition. Not going to do DMPCs of them for sure. I'm thinking of druid for some resists and heals, barbarian for net tossing and trying to hold the dragon down and ranger to help barbarian and throwing tanglefoot bag or two at dragon. Party should do the rest and should they get hurt badly, I'll let the dragon escape (most likely going to be badly wounded too) so they can track him down a bit latter and face him with a new strategy.
So the last question is: What music do you recommend for dragon slaying action?
And as bonus (if you are bored), names for the three NPCs? Probably going to make druid a female half-elf, ranger is going to be human male and barbarian is going to be dwarf male.