My house is one level with attic and crawl space, it has one bathroom
only, but, my master bedroom has a "room" that has a sink, and a door
that also lead to the bathroom.
My plan is to convert this sink room to a full bath room. the existing
sink and cabinet is rather large and I could see a small vanity,
toilet, and stand up shower can be added to this room making a 2nd
bathroom.

The question I have is of course the plumbing, can the supply all tap
off the faucet of the sink even for the shower?

A fan and vent needs to be added, can it go up the attic and tee into
the existing bath's vent? The 2 fans of the new and old bathroom
would only be about 5-6' apart. That way I do not have to cut up the
roof.

I need some advice for the draining, I assume the sink right now
drains to some main line under the house. Would the toilet and shower
be all using the same drain?

Would I need to replace the drywall with concrete board?

Is there anything else I didn't think of?? any advice is appreciated.