> I understand there is no GUI Boot menu a la GRUB possible.

GRUB is possible on G5 Macs. The inofficial Debian PPC64 builds depend on it.

> Hardware upgrade RAM to 2-4GB if possible for MacOS and Linux

2.5 GiB is the maximum for the hardware.

> SSD that can work with old SATA? Which models?

You may start from that thread:
https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=13116&forum=11

> is it 100e for G5 via PayPal?

"The price is 79 EUR for one laptop or desktop computer. Special pricing of 49 EUR per device is offered for EfikaPPC and Sam460 systems."
https://www.morphos-team.net/faq

> Linux Can read FAT32, can MacOS and MOS do it too,
> so I make 4th data exchange partition?

Yes, they can of course. For file size over 4 GiB and/or partition size over 2 TiB, I'd recommed ExFAT or NTFS-3G, though. I'm not sure about ExFAT for PPC Mac OS, but NTFS-3G exists for it.

> will encourage DJ Nick to register MorphOS on his SAM460, deleting OS 4.1 :D

There's no need to delete OS4 in order to use MorphOS on the Sam460 :-)