If another effect later causes the artifact to become a creature and sets its power and toughness as it does so, that creature will have that base power and toughness; it won't be 0/0. Notably, crewing a Vehicle does not set its power and toughness, so a Vehicle's base power and toughness remain 0/0 if you crew it.

The effect of Workshop Elders's triggered ability lasts indefinitely. It doesn't wear off during the cleanup step.

An artifact that becomes a creature due to Workshop Elders can't attack unless you've controlled it continuously since your turn began.