iPhone 6S Refurbished: Touch ID not working and Unexpected Shutdown, what are my options?

Going to be a long one but I hope someone helps me here.

I bought a refurbished iPhone 6s a week ago and the Touch ID doesn't work outright. I get a "Failed: Touch ID could not be setup". I've restored the software to 10.3.3, 11.1 and now 11.1.1 and nothing, same issue again. Didn't think too much, bit of a pain but said I'd get it fixed whenever I have time.

Now, I've encountered another major issue that makes me need to replace it now. The battery now dies at anywhere between 20-40% and refuses to turn back on. If I look up the IMEI and Serial Number, it was refurbished by Apple in March of this year but the warranty expired in November of 2016. As with the Touch ID issue, I've restored to stock multiple times and never restored from backup to see if that affected it but no, same issue, every day. I'm planning to go to the Apple Store tomorrow in the UK, I really only have a maximum budget of about €200 (£176) to spend on a repair or replacement. It was expensive enough for me buying the device a week ago and I really can't afford spending practically the same price again just to get Touch ID back and a working battery, at that point, I might as well have bought the iPhone 8 unlocked. A friend of mine looked at the phone and said it uses a Samsung chip, if that makes sense to anyone? Does anyone want to give me insight as to whether I'm eligible for a replacement/repair either free of charge or for a suitable price I can afford?

I've contacted the previous seller, they wanted £400 to either repair or send me a replacement and wait a week or two but that's not feasible as I need a phone for work and I paid £350 for the phone and simply cannot afford repair or replacement costs that go into the £250-300 range, Christmas is coming up and I can't afford it

Hope someone can help, thanks guys!