Recently I bought a sub-sales, and starting planning for the design.

As per understand from lawyer, it might take 4-6months before everything complete and able to get key. And after get key, still need to perform all those defect checking.
Needed advice for those experience, how soon we need to start approach interior designer? should it be after getting key and defect or I could survey now?

Any tips or common area to aware of?

More like actually decide on the style you want, so easier for you to communicate with designer or contractor. (can see those interior design facebook etc)

After decide the style, you can start to see how much budget you can afford and how soon you want to move in.

In between, can also start go window shopping in furniture expo to know how much furniture cost etc.

For defect checking, it usually take a while for developer to rectify. So, usually you can start engage with interior designer when all issues are rectify. In the mean while, you can just have a quick chat with designer to know about their costs or the style you want. Not need to rush to lock the style or designer till you are comfortable.