If you want to just use waze/spotify, you don't need mirror link. Just Android Auto. Cheapest Android Auto supported HU that I bought was this : https://www.lazada.com.my/products/i2850921...ls.item_title.1
Note that this also says that it supports mirroring. but honestly I don't use it because Android Auto is more than enough

1 - Yes Saga FL 
2 - No casing. Any 7" player SHOULD in theory fit
3 - Yes, I need to DIY. The original player has a curved face plate, while 3rd party head unit has a flat one. So you need some cutting the plastic front to make it fit. Also the aluminium bracket needs some loving and tender hammering. You also need some ISO extender and antenna wire extender (both very cheap) since the origina proton head unit has these cables extends into the back, while third party ones are built into the body itself, so the original cable might not reach it. Luckily Proton uses ISO connectors so you won't need socket converters.
4 - It works well. For an RM200'ish head unit, it detects my phone and runs android auto without fail every time. My only issue (and this is a BLM/FL/FLX issue) is that the radio placement at the bottom means you need to look down to use it. Which is not ideal if you are running waze/google maps. There are some shops that offers modifying your Saga BLM/FL/FLX dash, so your head unit is on the middle of the dashboard instead of down below so you can see it better. But I am not keen on drilling my dashboard to do it 

Yep. They should have a better idea on to install it nicely instead of how I did it 😅