The problem with Macbook Pros is that newer models are in many ways inferior to their predecessors.
For example, my 2011 i7 17' MacBook Pro (that I'm typing this on) has 3 x USB 2 (A) ports, firewire 800, Thunderbolt, display port, gigabit ethernet and a PCIe express slot.
Not to mention a nice large 17" display, an optical drive that burns CD's & DVD's plus user installable RAM and replaceable SATA hard drive.
It's currently installed with a 2TB SSD and 16GB RAM - runs great! :)
Move to a 2015 MacBook Pro and the Firewire is gone, ethernet is gone, optical drive is gone, the 17" option is gone and there's only 2 x USB 3 (A) ports and 2 x Thunderbolt 2 ports which aren't really usable for anything except for plugging adapters in to give you back the ports you've lost!!!!!
Try finding a Thunderbolt 2 hub....it's not easy. They are rare and they're expensive.And a Thunderbolt 2 external hard drive...that's like finding a winning lottery ticket.
So all added up you're losing a lot of flexibility to get a little speed bump, better graphics and the ability to run the latest Mac OS.
The only connection that the 2015 MacBook Pro has that my 2011 does not is an SD card reader, so it's hard to recommend a MacBook Pro to replace one from 2012 because in many ways, the older models offered more.
That said IMO the best value second hand MacBook Pro now is the 2016 (13,1 or 13,3) model because it's the cheapest model to come with Thunderbolt 3 which uses the ubiquitous USB C connector, so there's actually a plethora of inexpensive hubs, external hard drives and pen drives that you can plug into them.
Also for around £30 you can get hubs that'll give 3 x USB3 ports, gigabit ethernet, SD card reader, HDMI, and DVI (basically the connections that you'd expect any 'pro' spec laptop to have.
Apple (being apple) fitted most with ridiculously small storage (256GB), but if you hunt around you'll find models shipping with a 1TB SSD and I recently purchased one such model for £1250.
The keyboard isn't as good as the one on my 2011 17", the Touch Bar has little use and you'll need that hub I mentioned, but or the most part it still seems a decent laptop.
I doubt very much it'll last as long as my trusty 17" MacBook has, but then Apple don't want their gear to last anymore.
Built in obsolescence is their new mantra it seems.
Anyhoo - 2016 (model 13,3 15" or 13,1 13") are the model I'd recommend looking out for.
Good luck! :)