I would change the belt at least.
As mentioned, you won't be able to tell if the seats were changed unless you pulled the head... and even then, unless you have the documentation, I would assume it hasn't been done.
And yes it does depend on the damage. it will destroy a single piston at least. I actually drove mine into the shop to pull the engine. Later, I pulled it apart and was shocked to see only about an inch in height of the piston left. Others have had a rod through the block. The other pistons might get some dings in them, but as long as the top ring still moves freely, it can be reused. The ring was smashed (compressed) into place on the damaged piston.
Mileage? that depends. greatly.