I don't kown about the price, but I think if the company is helpful, it will take about 10 to 14 days.
1. As you said 樓下話滴水, you better take care with the waterproofing.
2.also better not to install a bathtub unless you really want it.
3.ask several company to quote the price. don't just pick the cheapest one. ensure you know what they will provide and must record on the "contract". if possible record the LD, e.g. $200/1 day delay. ( to control the completion date of the project)
4. take care with the waterproof around teh window frame (if any).