GPU pricing is insane right now, so you’re definitely better off buying something pre built. Based on the following experiences, I’d say something with an Nvidia GTX 1650 or 1660 would be a minimum based on current offerings.
I run everything on an old custom built gaming PC, it’s got an Intel i7 3770K paired with an Nvidia GTX 970. This runs Zwift acceptably well for me at the 4k setting, maintaining close to 60fps but often dipping down to the mid 50s. It’ll run TR, Zwift, Zwift Companion App through BlueStacks and stream music or videos all at the same time without missing a beat.
For reference I’ve tried it on an Dell XPS 15 from 2017 with an Intel i7 7700HQ and Nvidia GTX 1050, it’s pretty choppy even at 1080p. This is probably just a limitation of the GTX 1050, so I’d avoid anything at that level.