What brand for mist coat?

You do not need a so called mist coat with modern day emulsion paints as long as you use matt paint for the first coat. Back in the day when emulsion was new it was not very good so decorators use to say just give it a hit and miss coat meaning dont be too particular over the years this has become called a mist coat. Two coats of a properly applied matt paint will surfice.

You don’t need to use a brand name emulsion for a mist coat, just get the cheapest, that’s what I do, that’s what I’ve done for the last 45 years,

Most emulsion paints would be ok as long as they do not contain Vynal and the emulsion is thinned down Wicks also do an emulsion especially for new plaster

Depends on your budget. You can get dedicated primers for new plaster these days but they can be costly. If you have some emulsion to hand and it has vinyl in it (Dulux vinyl Matt for example) then thin it down with 30% water. If you're buying it then go for cheap contract Matt. Once the plaster is sealed it may need rubbing down and spot filling so give it another coat (or just spot prime it) before putting on the finish coat