I use both Debian Stable (sarge) and testing (etch) on several computers. If you want a stable platform and you can live without latest kernels and packages, then go for sarge. Etch can be a pain in the ass sometimes, because it is updated constantly and sometimes things get broken and you'll have to fix them. I think it's better to use Sarge unless you're already quite profient with Linux.
And now to the install problem...
I guess you could create a driver disk with Marvell drivers on it, but I'd simply grab some old (but supported) PCI network card, plug it in and install sarge through that. Then, when the system is up and running, I'd install the Marvell drivers. I'd also keep the old network card inside the computer, just in case.