Hi folks, thanks for the Infos. Especially @le51
In the first step i have a raspi5 and the dac8x. Also a metalcase for the official 7" Display, but not the Display. This would be a Touchscreen only solution. But the cheapest and maybe a good start.
In the local classified is a V3 zynthian for sale where i can replace the raspi (with the help of a iron saw).
I also had the idea asking @jofemodo to buy a V5 kit without the mainboard. But this would have technically issues as i know now.
I will think about it and have also time at the weekend for this topic.