I agree with @de0u, you don't want an outdated experience and undermine your security. Plus you're gonna have a hard time finding and installing GOS on older devices, as the official web installer only provides the phones that are currently supported.
What you can do is look for a good deal on the used market or trade-in options. With a bit of research you'll be able to get your Gen 6 or newer test phone for under 100$ because it doesn't have to be in mint condition.
If you decide that GOS is the way forward, I highly recommend only looking at Gen 8 Pixel devices or newer, because you'll get so much more with them (7 years updates, hardened security, desktop mode, less problematic hardware etc.).