I would do the move first (less data to move). If you have a lot of Steam games, you'll have to do the "manual" move as described on that Steam Knowledgebase page linked above. If you don't have but Silent Hunter with the Steam, it is probably easier to un-install Silent Hunter and then Steam. Re-install Steam, but tell to install in a folder of your own creation, such as "Games" or something you'll remember. That will then put the Steam Library folder in that "Games" (or whatever) folder. You will still see parts of Steam in the "Program Files (x86)" folder, but the Library and the important parts are not. Now when you go to install Silent Hunter, you should be able to put it in that "Games" (or whatever) folder, then do your modding. It would then be in "C:\Games \Steam \SteamApps \common \Silent Hunter...", ready to be modded.
No matter which way you go though, just remember that Steam is slow on the install or move. Whichever you do, it will take a long while, so brew a spot of tea or a pot of coffee, and get ready for a long afternoon / evening. Good luck!
