Hi Marco,
I extract the drivers from the *.cabs (e.g. http://en.community.dell.com/techcenter/enterprise-client/w/wiki/2180.latitude-e6320-windows-7-driver-cab.aspx). In there are all possible drivers, even if there is a component missing in our specific configuration. The templates stops, because a expect return code 0. I could ignore errors, but I thought it would be better to know when something's going wrong. 
Second question: I don't know. The point is, that there ARE no drivers for a specific model (e.g. D630), the standard Win7 drivers coming with the image are sufficiant. But if the folder C:\Drivers is empty, dism fails. The "workaround" would be not to expect 0 as return code. But maybe it's a good idea to simply create a DUMMY folder which contains a generic NIC driver that gets deployed during the custom HII.