Nick,
I would recommend raiding the corporate parts bin. A 400E brake conversion is something we have looked into before. The only question left unresolved is whether the master cylinder would have to be replaces as the twin-piston front calipers may be a bit more than the stock is up to. You could also try the big brake upgrade offered by Brembo but I don't know how well it would do as far as noise and whatnot. If you stick with MB parts bin raiding then I'd stick with OEM pads. For what you would be doing you would be quite hard pressed to overtax them and the downsides of going to an aftermarket pad would far outweigh the very small (if any) gains you'd get that .5% of the time. I've heard from several reliable sources that the slotted rotors can "groan" a bit under low-speed use. The cross drilled ones can have a bit of a rep for eating pads. (think cheesegrater) As far as brake lines go...as old as your car is I would replace them but you can go OEM just as well as SS lines. You would get the biggest gains from having fresh lines and the OEM's are certainly very high quality.

Hope this helps...Lee

PS if you are seriously interested in looking into doing a conversion for your 300E just fill out a PartsShop inquiry and we'll look into it.