My apologies to you Nicos.
Sometimes people on this Forum might not know a "jokey mood" from a wind up.
I, along with many others on here, take great pleasure in trying to pass on any advice or tips that might be of use to any poster. This might be just a few words as a short reply, or sometimes a more detailed one taking some time and thought with it, to try to make it as useful an answer as possible.. Your question/s to be honest looked a total wind up to me and I said so........ However, knowing what I know now, I will give you, along with my apology, a reply to your questions ......
If you've never plastered before, it will be very difficult to take on what you are planning.
PVA is always very important with this kind of job.
A sagging/cracked ceiling would probably be better being re-boarded, a "slightly" sagging one, might pull back up with more screws in, or if possible, get a few more bits of timber put in from above it to strengthen the sheets.
Always repair cracks in ceilings and walls if you are going to replaster them, preparation is always the most important job whatever your doing.
Never plaster over lining paper or any other paper, always remove it and prepare the wall accordingly.
And finally, the best thing to do Nicos is to get a plasterer in to give you some first hand advice. There is much to learn. Good luck with your job.
Roughcaster.