Well, I purchased the STG58 with the 21 inch barrel and folding bipod.
Most of the sa58s seem to be similar.
Do you want a side folding stock? That is something that cannot be changed later (to my knowledge at least, or not with a great deal of changes).
I personally think with the round, it does not make a lot of sense to get a short 16 barrel, though you will see a nice fireball out of the end.
They make some with a different top cover that has rail on it. You might want to get that if you want to mount a scope.
If you plan on putting a bunch of crap on the front, you can get a quad rail for it also, though I feel like the rifle is generally heavy enough by itself.