Yep, totally do-able, buy second hand/something that has a fault.
I bought a B450i/3500X/16GB ram/RX 580 8GB MITX 2nd rig off a mate at what he paid last year with a faulty gpu - £280, and just upgraded bits as I went - it lives in a SG-13 btw
So you shouldn't have a problem doing this with any spec build, just get a reasonable motherboard and a 'good enough for now' cpu/gpu and upgrade it over time once you're up and running?