My girls had the Coach package at ages 5 & 7 - hair extensions, make-up, nails, and they had an absolute blast ! Here's how I would answer your questions:
 I would highly recommend bringing your own dresses...no guarantees on that. We brought ours and had a mid-morning appointment...had them in a dress bag under our stroller til the appointment.
 Not sure how the in-park dresses compare in look, but I know they are pricey ! Try checking the Littlest Dress Shop online. We ordered a few dresses (just for in-Park wear) that were lightweight and well-made from this company.
 With waiting, changing, princess-ifying, paying, photos after in Castle Couture (awesome!), we were into the BBB for 80-90 minutes altogether. I would allow 90 minutes on a touring plan just in case.
 Bring your own brush, know that the look lasts all day, but be prepared with a change of clothes, the hair extensions were a big hit over the ballet-style bun....too tight !!, take lots of pictures, get the mini-photo shoot done at Castle Couture next door...if you have MM, it's included. Priceless !!
BBB was a highlight for us...hope it is for your family, too !!