Yeah, the Haunted House was a bit underwhelming. Neat twist though.
If I was to do it, I'd probably steal my top three favorite plot hooks from DMG 2, steal a dozen of the more colorful NPCs, and keep them in my back pocket for when the PCs go off exploring on their own. If the PCs opt to follow one of those rather than the U1-3 track, I'd either steer them back or let the campaign wander in that direction for a while, whichever seemed more interesting. I also might toss in another alternate place to explore if the PCs so choose. (Bone Hill is another blank-slate dungeon with a nearby town, substituting Restenford for Saltmarsh should be easy).
Anyway, I hope your group enjoys it!