Steam is not as bad as it once was. I have several Silent Hunter games running from Steam with no issue. You just CANNOT install the Steam Library in a Program Files folder, which is what the Steam install defaults to. If you have two valid hard drives, you can install Steam in its default, and then install Silent Hunter 4 on the 2nd hard drive in another Library folder, or just install it all in a folder of your own creation. You can change the path easy enough from "C:\Program Files (x86) \Steam..." to your own "C:\Games \Steam..." or whatever you want.
Also, just as an option, Ubisoft US has it "on sale for $2.50 for the Gold Edition at
Silent Hunter 4 DLC-PC - they are similar to Steam in having their own "starter" app that checks the licensing when you start the game, but it is easier to copy and paste the Ubisoft version to different folders for custom modding. Steam can be a bit fiddly about that if you don't do it correctly.
A disk is the best way, but you most likely will not find one of those, unless you find someone on eBay that doesn't realize what they have, and will sell if for less that $50US... There is one in England right now for $8, but shipping and duties will kill that, and then there is one in Spain for $35, and I have no idea how much shipping and duties will add to that... and that's it, two disks, both used... Do you believe them when they say "very good condition"??