Just moved from my 830 to the 1520 and it has made a world of difference to me. Other differences aside, the 1520 doubles up as a mini-mini tablet, which helps me a lot - in terms of reading, doing web-related things etc. The internals - better by far than the 830 - are robust enough and while I have no proof of any kind, which is only another way of saying that I am just being speculative, I think they will do quite well in a Windows 10 world till at least the 1stQ of 2016 at which point an upgrade will become essential.
Needless to say that I bought mine - on the recommendation of a kindly member on Reddit - for ?252 (including shipping) brand new, which made it a lot more palatable that I had originally imagined.
I would say that if you got a good price and the phone was either new or in very good shape, then buy it (and pray that the reported h/w problems do not plague you - I've had mine for 24 hours only and I am being optimistic about my device...why would I not be :smile
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Good luck!