Plenty of good advise offered so far . I would agree regarding work permit issues , be the project manager and you should be ok . Source and buy the materials yourself , every builder and supplier are well versed in the "two receipt trick" .
The price for basic building materials is pretty standard , your final build cost will very much depend on the standard of fixtures and fittings you go with .
Anyway , good luck with it & look forward to seeing pics once you get started .