I've done more drywalling than I care to admit, and this is all good advice so far. The mudding is tricky- be careful with it, since this will affect the look of final outcome more than anything else. Be sure to sand it smooth between each coat- and wear a respirator! That dust will kill you! Be sure to seal off where you are working from the rest of the house with a tarp, and use a shop vac to clean- don't sweep- that dust will get into the air, coat everything, and give your lungs hell.
As for covering patches over old horse-hair plaster, try to knock out as plenty of room around the bad plaster, and make room for the drywall. Use tape for the joints. Good luck, and be sure to take some pictures of the process!