It *should* be quite clear what's damaged, depending on how bad the damage is. It's at least semi-likely that the valve won't be sitting on the seat properly if the shaft is bent.
And the extent of the damage varies with each different engine - in some cars the valves protrude A LOT into the combustion chamber, and in some cars they don't. Some heads have a clearly visible "dip" in the head for the combustion chamber, and others have an almost flat surface across the head along the CC. Sometimes the valves are at massive angles to each other, sometimes not. As you can probably figure out, it really depends on an engine-to-engine basis, taking into account how the engine was designed in the first place.
Head gaskets? You'll have heaps of choices. Some are thicker (meaning you'll lose compression, and therefore torque down low), some are thinner (meaning the opposite effect - more compression, meaning more torque, and quicker to come onto boost). You don't have to spend a ridiculous amount, just make sure you're buying quality.