OK, finally it worked, a little bumpy, so here is my experience:
First problem came during the install of the Combo Update, it stopped with 2 minutes remaining, but packages seemed to be installed completely. Starting again failed, after a few attempts I was able to boot in safe mode. I ran the combo update again, same procedure, it stopped and I had to force quit. Nevertheless, it said 10.5.8 in the "about this mac" window.
So I booted again in safe mode and did the postinstall (chameleon, 10v extensions and dsdt.aml file, nothing else).
Now I have a it all working, except for sleep mode. I think this is because of usb legacy mode in BIOS ... got to disable it again ...