Few things to keep in mind:
1. age
2. you'll have to upgrade it to w10m via the insider route, but you'll be stuck with TH1 update
3. you're buying used so lifetime could be seriously reduced
4. apps, but if you already own a windows phone, this isn't news
Moving the phone unto the latest w10m builds (TH2) is possible but it will require some work and technical knowledge from your end. Also patience... lots of it.....
Another aspect to keep in mind is the hit or miss nature of w10. Some report stellar performance and others not so much, so the mileage varies by a somewhat large margin.