Let's start somewhere simpler.

Find an experience real estate agent.
I suggest you talk to him and whether he can give you suggestion.

I know, but don't underestimate what a real estate agent might know.
TS need to find an experience one who knows people.

It's very hot here in JB, I'll make my answer short before I run somewhere else.

Seek an experience property agent.
A good one would have contacts to a contractor.
Relaying your queries to him, he will give you a rough guide on the cost on your house.

The agent would also know mortgage broker and lawyer.
Where contractors have contacts to an architect.

Get your team together and discuss your problem.
Have your team help you.
Best if they know a developer and he is willing to mentor you through your endeavor. Good luck.