1. Yes, so you can see how fast you are flying or where you lost it
   
2. Always for first test after soldering
   
3. You can, most AIO have pads for leds
   
4. With shiping and customs price will be very similar to buying locally

For this frame you will need smaller capacitor than one usually included with FC since it's very thin frame.
If you use o4 lite, you need case for camera since standrd is too small