Knock off the bits of dry plaster, othewise you will be spreading a lot of plaster dust with the sanding. A dull knife or plastic spatula would do the job.
A sander with a bag is a great idea, if you value your lungs. It will come in handy for other things later. The lower price “handyman” model would do your job fine, you don’t need a “professional” model as you won’t be using it 8 hours a day. There is a shaped sander available, shaped like the bottom of an iron. The point of this can get into grooving in the skirtings. For very tiny grooves you would have to use hand-sanding paper, folded to be using the edge. They don’t need to be totally stripped back to bare wood.
Source: YAhoo answers