Lots of ideas floating around, this may help make sense of some of them.
The clear coating you are looking for is Polyvine Decorators Varnish. It comes in dead flat (which isnt dead flat!) satin or gloss. Make sure it isnt the Polyvine wood varnish which looks similar but isnt.
Proper tough paints I know are Dulux Diamond range, or Little greene Ultimatt (Intelligent Matt) or Mythic acrylic latex. Mythic is astounding, as it is scrubbable in the matt finish, which is actually matt too. You get what you pay for by and large.
On bare plaster you can use 2 coats of acrylic matt, first coat diluted by 15% with water. It is a bit / lot more expensive paint than cheaper vinyl matt paint, but a third less on labour.
having said that, on existing ceilings in good condition I have been assured that Leyland vinyl matt in white is reliable and economical, but other contract or vinyl matt paint, i am not convinced, but I am in a minority :)
If you have stains on the ceiling, I wouldnt muck about, just apply 2 coats of Blackfriars problem solving primer or mythic multi purpose primer, you will be amazed at how nice and white it looks and stays. leave about 4 hours between coats though, else the second coat will reactivate the stain.
You can make your own bathroom paint, add a sachet of Anti fungicidal solution to satin paint. The satin is the minimum sheen to choose, matt is too "porous" to resist the spores.
Synthetic brushes in acrylic or water based paint are a must. Damp them before use, clean them every few hours, wash them out at night, never in warm or hot water! I dunk mine overnight in a cleaner called Krudkutter, it keeps them immaculate. Picasso is the best brush for emulsioning. The Wooster kit I recommend is top of the range and will last you a lifetime, that works out cheap, if you paint remotely regularly.
I absolutely dont subscribe to the idea that DIY equals slapdash or shouldnt look good. With decent kit and decent paint, you can match the standards of a lot of average decs who wont go the extra mile because the price is too tight to do otherwise. The main advantage the average dec has have over you is speed and they should know instantly how to get over a problem, otherwise, if you have the time and a painty mind, go for it.
A lot of painting is over thought, trying to get the nirvana of cheap and perfect. If you want perfect, you need to invest in tools and paint and have a real talent and a steady hand. I get most things right first time, but if I don't, I dont have a "that'll do" attitude, that is the start of along slippery slope.
At the end of it, prospective buyers / the new people should walk in, see nothing but solid flat neutral colours everywhere, framed in nice smooth woodwork, and you will have a head start on the guys who slap on cheap coats and hope people are dim enough not to notice.