hai Izwan here,
to answer your question

1) its better to separate builders work and consultant works. Avoid people with interest....other than that, you can control your payment by having separate scope/ stages of payment and not park all payment to one company... they have cases whereby my client blindly pay contractor just because they say they can give you the sky and moon as well... so careful.

2) You can engaged a design and build company but make sure it is a binding contract which indication of a proper separation of scope of works and stages... dont find a company that simply put LUMp sum... 

3) you can engaged a project manager but depends on your budget.. if medium renovation i suggest you can monitor yourself......
how you monitor?  ask the contractor to come up with their master work programmed.... their schedule of works... but need to be detail lor.......

4) I doubt you can find the original layout plan from the developers since you mention its been 10 years... since you are doing the renovation, the architect submission to council and redraw back the layout plan... they will do measured drawings on site.....

5) You can tell which brick or which is concrete by looking at the beam design and colum position... the end beam will end mostly having a structure column... mostly la... but still need check.....

you can drill a small hole and see the colour that comes out... since its an old house...i assume they are still using clay brick... can see the colour if clay brick.... you can test the porous as well.. if easily drill and porous -of course brick lor... if not hard to drill ones- then concrete lor.....
hope this help
cheers mate,
Izwan
www.archidstudio.com