You can get a battery on Ebay for about a tenner. Its a quick job to replace. I’ve just upgraded to the 5x as I broke the screen (again) (32gb for £200 on ebay) and although its good, the camera is great. I don’t think its too much an improvement on the original. Its slightly bigger than the 5 and I still wish for a better battery.
However, in a few months the next Nexus phones should be released. Could be good to wait.
The Oneplus 3 also looks good for the money, though I always like the quick updates Nexus phones get.
Edit: The battery is easy to replace, stick a bank card in the side and wiggle around to pry apart. Then just take 6 screws out and swap over, the battery is lightly glued in place.