Do I get that right? A masterworked weapon gives me a +1/0 Bonus which does not stack with the enchantment bonus (that is always on attack and damage). So if I want to enchant my weapon I would pay (2000 gp) for a +1 damage only. So why would I use a masterworked weapon to enchant and not just a normal weapon?
A masterwork weapon is required
Only a masterwork weapon can become a magic weapon, and the masterwork cost is added to the total cost to determine final market value.
Armor to be made into magic armor must be masterwork armor, and the masterwork cost is added to the base price to determine final market value.
Once you enchant your weapon magically, it is now a Magic Weapon, which comes with all benefits attached to it, like allowing your to hurt incorporeal creatures and bypassing damage reduction X/magic.
Yes. A masterwork weapon is so finely crafted and balanced that it makes it easier to wield. An enchantment bonus adds a extra edge on that finely wrought blade so it bites deeper.
You cannot enchant a regular sword. It must be masterwork first. So your price would be a normal sword price, addition for it being masterwork, and add an enchantment on top of it.