The reason that MB never put other than rubber mounts on the 300SE is that it is a smooth running ca (when everything is to specification). Liquid-filled mounts usually are required when there is some vibration due to primary or secondary imbalance that can't be engineered out easily (actually inexpensively).
Your other question was do you need an under-hood cover. The answer is that, yes you do, to keep the "noise " of the engine quieter. Remember, MB are supposed to be "luxury" cars, and hence quiet. This under-hood mat is very inexpensive, available from your local dealer, and yes you can install it yourself, as it just glues up using 3M spray adhesive, which the dealer also sells. Just makle sure that you do a very good job of clening off any residual adhesive, and that you spray both the underhood and the mat sufficiently to bond. You will have to spray both first, then carefully prsee the mat onto the underhood, and you have a quiter engine. Try it, you'll be amazed at the difference.
New door seals will help only with wind noise, not engine noise......Andras