> I would like to buy a New compatible PCI-E GPU for use with MorphOS.
> Preferably it should work with AmigaOS4. Used cards are off limits.
> What do folks recommend?

In the EU, you can currently get new from online shops these compatible Radeon cards:

HD6450 for 40 EUR
R5 230 for 42 EUR

MorphOS currently supports these with 2D acceleration only (no 3D, no overlay). There's also no 3D support for these cards by OS4.

> Will MorphOS work with the RX580 [...]?

Not as of current MorphOS 3.13.