Stick with Mecgar magazines, for Lugers or anything else, as many manufacturers use Mecgar as original equipment magazines. I have mecgar mags for many of my semiauto pistols and have never had one which would not work.
I too, am new to the Luger game, and I know how it can be just starting out. I am sure that some of the advanced collectors/historians/authors in residence here can tell you about the markings on your new shooter. As for grips, try Sarco, Inc for parts as they are one of the largest military gun parts surplus vendors in the nation, and they are great to do business with. I ordered a new set of walnut checkered grips for my shooter as well as two magazines so i would not be putting wear on the magazine which came with it. Sarco has barrels (4") also, but I believe they are in the white and would need bluing.
www.sarcoinc.com