Hi, Mary. This seemed to be a pretty easy do-it-yourself project until you got to the part about the 10 foot ceilings. This would indicate that the house is old and the framing may not be what we would consider as standard today. I would consider hiring a carpenter to do the new framing and then you can install the new drywall. The framing should go very quickly and not be a big deal, particularly if you have the right tools. The drywall is easy to cut and apply but it takes a long time to do with the waiting for drying, etc., and you don't need much in the way of tools. A pro is not really going to want to do a small drywall job because it does take so many visits. By the way, if you have an older house, there is also a good chance that the walls are plaster.
Watching a couple of home improvement shows, like Hometime, on TV about the subject will tell you a lot of what you need to know about cutting, installing, mudding and tapeing drywall. It's a heck of a lot easier to understand if you watch it being done than to try to go from written instructions. Watch particularly how they do corners. You will probably have 2 outside corners to do. Sanding drywall mud is always a mess so be prepared. You will have to modify the base and probably crown moldings also. Hopefully, you have a source for matching moldings.