I have a DSA STG58 on the way, and am already thinking of different furniture. Does the DSA have US made furniture as part of the US parts count, or does it use the original (foreign) furniture? In other words, can I install foreign made furniture or must it be American made?

If you are buying the StG-58A, then your buttstock and handguards are Austrian, and your pistol grip is US made.  As long as you keep the US pistol grip, you're cool, you can swap out the buttstock and handguards to your heart's content.

The SA-58, with the Badger barrel and lightweightlower, have more US parts, including the handguard.  You can install a complete foreign stock set on the SA-58 because of the US barrel, US cocking handle, and US lower receiver.

Here's my DSA StG-58A with a wood stock that I got from DSA's European walnut set:

That's exactly the stock I was thinking of getting!!!

The wood humpback stock is much more comfortable to me than the polymer Steyr straight stock, and I also think it looks much better.