I have a 2014 retina Macbook Pro with 16 GB of RAM and it is holding up very well. Maybe look for a spec'ed out used 2014/15 edition? Battery replacement kits are available for around $100 and Apple will do it for $150 (I think...) which should buy you a few more years.

The upgrade to a retina display will be nice if you're using a 2012 laptop.