What a mess! The bad plaster wall repair job by the previous owner has left you with a real predicament. You have two options:
1. Spackle - Build-up the difference between wall thicknesses using several layers of spackle. I would start with a 4 inch spackle knife, then switch to a 6 or 8 inch blade, and then a 12 inch. Each layer of spackle should be put on as thin as possible. Sand between coats. The idea is that you need to taper out the spackle from the botched plaster wall patch to the finished wall.
2. Rebuild the wall - For this you will need to remove the trim on the doorway, remove the bad patch, then frame or trim out the wall so that the new patch will be flush with the old wall. Consider using 3/8" Sheetrock if 1/2" is too thick. You will probably take some of the old wall with you when you cut out the bad section. You will need to decide whether you want a horizontal seam or a vertical one, and then cut accordingly.
In either case, the plaster wall repair isn't going to be as smooth as you'd like it to be so I'd opt for a textured paint finish. There are some beautiful plaster finishes available at The Home Depot and any kind of textured paint hides many sins - including those of the former repair person!