Buying from China could save you a bundle (even after additional costs of importing such as Duty and GST), but would probably require considerable time and effort to research and organise... you would want to make sure you get exactly what you pay for. There are many potential pitfalls, including what happens if something goes wrong? The two Chinese companies Ive heard of that you may want to look into are Shanghai YY Construction and Feilong (just Google them, they both have websites).
Not really sure about China, but know a good company https://www.blueskywindows.com.au/windows/ that installs double-glazed windows of very good quality. They are completely soundproof and also provide heat preservation. I think it is a good alternative to what you are looking for.