Can I just do L-box and lightrought as as my apartment is around 10' in height only?
If yes do I need to do a skim coat on the center part which is not plaster ceiling to make it have a smooth feeling like plaster ceiling?
And usually does the quotation including painting work?

I didn't use primer for my plaster ceiling, I just painted it with 2 layers of nippon matex ceiling white.
according to nippon website, 10 - 13 m² per litre per coat. I bought a 18L at around RM60, used half to paint 400 sq ft of plaster ceiling.