Firstly, you need to alow the plaster to dry out completely before you start painting.
Depending on how fast the surface absorbs the water will really depend on the ratio of paint to water for your mix. Start with a 4:1 mix (4 parts paint to 1 part water) to see how fast the water is absorbed and add more water to a max of 1:1 if the plaster does not absorb right away.
In terms of paint, most brands are pretty good (Dulux, Crown etc...., different people have their own personal favourites) but in respect to the bathroom/kitchen, it's best to use a specialist kitchen/bathroom paint due to the extremes in temperature and moisture.
No problems - in terms of the paint, just make sure that it is a specific kitchen and bathroom paint with mould inhibitors that can deal with the extremes in moisture/temperature etc….
As said, most tradesmen have their own particular favourites that they have had good results with in the past. It may be best to go with a specific trade version of kitchen/bathroom paint as the trade versions tend to be slightly better than their straight domestic counterparts.
May be worth a trip down to your local builders merchant to see what they would recommend and what is popular with the tradesmen that they supply to get a better idea
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