Andrew Leavitt, writing on Medium:

You need only spend $171 for your anemic 27” iMac to run macOS Sierra, boot in seconds and open applications in a blink. Or you can buy a used one and upgrade it for just $500 starting from scratch. You really only require the latest models if you are working frequently with 4K video.

A machine that was originally built in 2009 can easily perform past another 5–10 years with a few straightforward upgrades. CraigsList has these computers lightly used for as little as $325 if you are patient. For another $171 in parts you can rock a machine almost matching today’s specs. And if you are presently struggling with an older iMac the decision is even easier (c’mon, it’s less than $200 and just an hour of tinkering). It’s fun and you might even learn something. Keep in mind that a new 27 iMac has a base price of $1800 (that’s with a 5K screen but no SSD and only 8GB of RAM). Boom, you just saved $1300.

We’re at a unique point in the upgrade cycle, with machines like the Mac Pro and MacBook Pro long overdue for an update, yet still selling at a premium. The opportunity is there to buy a used machine and upgrade it.

One path to explore is to build your own Mac from parts, a so called Hackintosh. Before you go that route, read this article. Then make your way through Andrew’s post and consider starting with a used iMac and retrofitting it. That’d be my choice.