Lukas Renovation

Go to timber shop they will cut to size any 25 mm board and after fit it or you can get any timber and cut a few pieces join them togheter and fit into place.

Depends how bad is it look... if the plaster is wet and crumbling better is remove it from wall but you can try wash mould and after drying wall with any heater if look alright after this paint with zinsser shelack bin primer 2 coats and after you ready for any filling or painting. if plaster need to remove you can use after one coat plaster british gypsum up to 13 mm deep if is more you need hardwall and multifinish after depends how big area... small area can use joint skim filler toupret after hardwall. any plaster need to leave for dry longer :)

Try zinsser shellack primer bin

Mdf board is not really good for bathroom you should have plywood on any backerboard or cement board suitable for tiles. but if you want to do on this board buy sbr bond on amazon paint it and after paint with membrane waterproof may this will work.

It is easier to paint frame first and any touch up spot if damage when hanging door.