No official agent from Xiaomi for Mi Box. But you should be fine to buy from Lazada and Qoo10 -- just need to check the reviews of the shop. Mi Box S build quality looks pretty good and no reports of hardware problem from what I read here and elsewhere even though I myself do not have a Mi Box S.
As for something in between Mi Box S and Nvidia Shield TV, unfortunately no if you need the Chromecast feature. You can say that apps like Starhub Go, Viu Premium, SingTel Cast, SingTel TV Go, Toggle and HBO Go, are the most popular streaming apps in Singapore. They are all designed for phone and the best way to get them on TV is to use Chromecast. Only devices like Chromecast Dongles and Official Android TV OS device support Chromecast function. Other than smart TVs like those from Sony, the only commonly available Android TV OS devices are Mi Box S and Nvidia Shield TV and Starhub Go Streaming box.
Starhub Go streaming box does not support Netflix and it is more meant for users who want to subscribe to various Go packs.
On the Netflix front, cheapest device to support Netflix 4k is basically Mi Box S or Amazon Fire TV Stick 4k. The other devices like Apple TV 4k, Nvidia Shield TV and Xbox One, Play Station 4, are all much more expensive.
So in the end if you need either the Chromecast or the Netflix 4k feature, you do not have many choices. Mi Box S is the cheapest with these two features and the other is Nvidia Shield. Nothing in between.
If you do not need these two features, you have many choices.