L520 and actually most of the cheap Lumias are/were great for the money they cost.. I tried to find similar Android in the same price range but never found one
Thousands of people touch these phones before they are released. If it would actually be released in February this would have already surfaced all over in the internet.. even for christmas 2016 release
Re: should i develop an app for windows phone 8.1 or start with wm10?
Well 7 pretty much is, but I personally have been getting almost as much downloads for 8.0 than for 8.1
Re: should i develop an app for windows phone 8.1 or start with wm10?
As there is still a lot of WP7 and WP8.0 you should start without ignoring those users if it's possible. Eventually you can switch to 8.1 or WIN10.
Universal apps, so basicly it should be pretty straight forward except you probably need an actual device to get something proper done. SDK should be released whenever the device is released.