About audio: yes, solving the audio problem without loosing PCIe of the 3D expansion is one issue. Nice idea about that SPI. Perhaps audio via USB could be another or Audio via localbus connected xcore.

"spend quite some hours reading a lot and calculating this and that"

I think I'm going through same hobby engineering/research fun ...

For me, I would like the HW to be a little faster than my curent SAM440@667Mhz.
(goals 1:learn HW 2:do some designing 3:try to make/get a prototype & OS running on it 4:see if I could manage to find means for actual production run 5:provide affordable opensource PPC HW 6:keep it PPC AmigalikeOS compatible, just in case if one day....)