I'm in this position too. Mine (early 2013) is still working perfectly and does everything I need for both work and, albeit a bit slowly at times, editing photos.

I'd have bought a Windows machine for the latter requirement but they no longer appear to sell a _Professional_ OS for non-enterprise customers.

> my 2012 model is still working fine but I'm guessing Apple will end its support by 2020.

Even the mid 2012 unibody MacBook Pro can run Mojave, which should receive security updates until Fall 2021, assuming Apple continue to support each macOS release for 3 years.