I'm in the same boat at the moment as I'm seriously considering an upgrade for my tired old screen.
The best place to look is here:
http://www.whathifi.com/news/boxing-day-sales-best-tv-deals-smart-hd-4k-tvThere's loads of reviews on what hi-fi and I've always found them to be really good. If you're not in a rush then buy the magazine for a few months until you get comfortable with what's out there. If you need one now then the reviews in that link will have to do.
Regarding screen size 50" is pretty big but if you've got room then go for it. To give you an idea this is what a 50" screen looks like on a 4m wide wall:
Click to see full-size image. |

Bear in mind that new screens have hardly any trim around the edge so a modern 55" screen would probably appear about the same overall panel size and a 50" would just be the bit within the black trim.
There's no real need for 4k at the moment as there's hardly any content to play through it. Most of what is available is on Netflix or youtube though so in your case it may be worth it. If you're not getting a 50"+ screen then you're not going to need that many pixels anyway.
Curved seems to be a gimmick. There doesn't appear to be any real use for it other than to look cool. I might have to get one myself though because they really do look cool.
A smart TV will do lots of cool online stuff although not all smart TVs do Netflix so look out for that. I think Samsung and LG both have Netflix within their OS but not many of the others. There's other ways of getting Netflix such as hooking up your laptop but you're much better off keeping it separate in my opinion. Many have wifi for streaming but you may suffer from jitter if your wireless connection isn't great. Better to have a wired connection if you can and also you'll need 30Mb/s to stream 4K from what I understand.
It's a huge minefield out there so very best of luck. Whatever you buy be happy with your choice and don't look at TVs again after you've made your purchase. It won't be long before you see the same TV at half the price so you'll just upset yourself........