If the system is evacuated and then flooded with dry gas (such as nitrogen), you won't be adding much water to the dryer and reusing it is almost certainly fine, if the work is done quickly. But if you're taking it apart and replacing any other system seals, replace both the dryer and TXV.

If you're sticking with R12, you can use green or black o-rings. Be sure to use the appropriate lubricant for R12.

The service fitting cap should seal the low pressure port. You can replace the valve core, but also ensure that your cap seal is in good shape.