Which MacBook Pro should I buy

2012 13” or 15” (Standard screen or HR AG). Just make sure you change the hard drive cable when you get it (or shortly after) so it is ready for a SATA III SSD - the original SATA II cables in many of these systems are fine with the original drive, but once you replace it (which you WILL once you swear at the original spinning hard drives they usually shipped with) then it will create a boot problem; it may also prevent future problems these are infamous for around this cable by getting ahead. They have no problems with SATA III drives unlike your 2010 13”, but the cable is the sour spot. With 2010 machines the upgrade sour spot is the MCP89 (320M northbridge) chipset.

If you want something a bit newer, find a 2015 rMBP with 16GB of RAM (soldered; no upgrade path!), and preferably a 512GB SSD if possible - 256GB is fine, but the drives are expensive enough you should try and find it that way. If you cannot afford the factory 512GB system, it can be upgraded; just bear in mind the cost of the upgrade will bring you to a similar price point as if you just bought it that way and many of the 512GB machines have an i7 instead of an i5 (common in 256GB systems). You also need to watch out for antiglare coating issues to a point since it’s a known problem on the rMBP 2012-2015 series with many of them being close to no longer being covered. Nearly every 2014 has an issue, the 2015's seem to fare BETTER but they still suffer. If it's out of the way live with it otherwise it gets annoying until stripped off or the LCD is replaced.