Another thing to be aware of is that different barrel lengths require different gas tube, gas piston and piston spring (I would imagine) lengths. People have shortened their long barreled rifles, but there are a lot of considerations when doing this due to the gas system.
As far as the DS Arms STG58 vs SA58... I could be wrong, but I doubt that the SA58 is completely free of refurbished / refinfished military surplus parts. I recently bought a STG58 (the parts kit rifle) and I love it. Many of the "important parts" are new anyway, for the sake of legal compliance. The receiver is new, the Hammer/Trigger/Sear assembly is new, muzzle brake is new and a few other parts. The majority of the used parts didn't look used and the gun was refinished nicely. Here are some pics of mine...
As it shipped to my FFL in New York:
With new handguard and proper magazine:
- Pete45