For the Go pro or Mobius a 450 size quad is a good size.
If you want to get in fpv quickly, and don't want to break too much, then a 250 quad is great.
You can buy the kit for a 250 quad for $100 Just need RX and battery.
But for LOS flying its a bit small. Once you are more than 50m away you will loose orientation.
450 size, there are many choices.
I have the QAV500. Very nice machine, but fragile if you think you're gonna crash a lot.
I well built machine would be something like the TBS discovery. Strong frame, cheap easily replaceable arms.
As far as electronics, I can advise you when you decide on a frame.
Good luck.