I got RM2.50 for flat and RM9.00 for L box, exclude painting..

Ts, if rm 4 for board thinness more or equal to 8mm and above then it should be fine. Looking at ur covering area are big, surely u should nego to around rm3~5. If u orang kaya then tutup mata sajalah.

On painting, no need glossy paint for ceiling, trust me,it will look funny or awkward.1 layer of coating and 1 layer of  high ent paint will and looks perfect edi. Save the glossy one for ur wall.

During my time, i had done some skim coating due to my unit wall are wavvy and uneven (developer contractor sucks!),paint the whole unit for 3k only. Covering 1000sqft

Pm me if u want my contractor contact. Super reliable person and good workmanship.