Seems like a decent price, GST is included in the price but Banggood will charge you for postage and insurance. You don't need to pay for insurance if you don't want to.
This is the global version and according to the Poco website it should have band 28 so don't get your knickers in a twist.
MIUI is what it is, i currently have a mi mix 3 5G and the MIUI is okay its stable and offers heaps of features, the camera app is good but id probably install Gcam on the X3. I have Gcam on my mix 3 and some shots are better but the stock camera app does well.
Most of you will probably use a custom rom any ways i am going that now with the mi mix as Xiaomi stopped offering updates.
Mod: $A price in title using today's mastercard rates, paying in USD.