Usually for ID + 3D with all the detailed drawings + M&E/RCP about RM4/sq ft build up. However this is always negotiated upon and depends on your scope.

This pricing usually only for drawing works, project management will be different. Case-to-case basis but not all ID will have this option, also depends if ID are Design & Build. If they are, then they will be willing to lower their design/project management fees when you take up their renovation package.

It's easier for some owners as you just deal with 1 key person/company rather than a whole bunch of contractors. If someone f*cks up you just shoot 1 guy. They're responsible for getting things right for you.