As mentioned elsewhere, the Lumia 1520 (so far) is the most future-technology proof Windows Phone out there despite it's age. However, Microsoft hasn't confirmed that the L1520 will get Windows 10 Mobile through an official upgrade - this doesn't mean that it won't, it's just not yet confirmed; besides, there's the Technical preview program if you're comfortable with it.
There are few things you may want to consider:
If you've got big hands, then the L1520 should be comfortable to use. I'm a pianist with a 10-inch finger span, but I still find the L1520 a bit awkward when I tried it in the store; probably would have just taken a few moments of getting use to.
If you're comfortable with the idea of not keeping your phone in your jeans pockets, then the L1520 will do - there have been many reports of screens cracking because of warping while inside tight jeans.
Finally, if you're comfortable with the idea of buying a second-hand phone, with an unknown history (we don't know if it has been dropped, repaired, etc... all we have is the word of the seller), then go for it.