I understand your concerns although even old original will be better than any aftermarket battery for a not popular 5 year old phone. What I would also do is check your previous battery with Nokia Energy Profiler. You can "measure" the capacity by charging your phone fully, disconnecting from charger and start running the profiler. In the upper right corner it shows how much Ah has already been drained.
My battery after 3 years had 1200mAh and the new original I bought had 1400mAh - even though it was older than mine.