I live in a house built in the 1930's where most of my ceilings have blown plaster so I always use a layer of 9.5mm drywall, it is easier than plastering onto the existing plaster and I don't have to hack off the old plaster which makes a mess and takes a lot of clearing up. Once you have the drywall on the ceiling you have the option of plastering that or just filling the cracks and nail wholes, which is a whole lot easier if you don't knwo how to plaster, use taper edge board for an easier finish on the major joints. It is a two person job, three is better. Getting a good finish with plaster takes skill so you might be better off calling a professional plasterer, if you have done all the preperation it shoudl not cost that much.