An artifact that becomes a creature due to Tezzeret’s Touch can’t attack unless you’ve controlled it continuously since your turn began. It doesn’t matter whether or not it was a creature at that time.

An artifact creature enchanted by Tezzeret’s Touch becomes 5/5 instead of its normal power and toughness.

A Vehicle enchanted by Tezzeret’s Touch becomes a 5/5 creature. Crewing it won’t change its power and toughness.

If the enchanted artifact is an Equipment, it becomes unattached and it can’t be attached to anything for as long as it remains a creature.

If Tezzeret’s Touch becomes unattached from an artifact that’s attacking or blocking, that artifact will be removed from combat unless it’s normally a creature or another effect such as a crew ability is also making it a creature.