Newbie Samsung RS21 Help!

Hi – I know that this topic has been covered extensively already but I was hoping someone might be able to help me out here.
The house we bought came with a used RS21wcsw. no receipt, serial number – 855443cy100033 B.

Anyway I’ve started to get the familiar problem with the temp sensors and lots of icing up inside the fridge.
What I’d like to know is:
1 – Is the 5 year watchdog warranty valid if I wasn’t the original purchaser?
2 – Is there anyway I can date the unit from its serial #?
3 – If I don’t have any kind of warranty on it is the fix worth the £100 odd quid in the long run or would I be better off saving up for something more reliable?

Thanks in advance
Al