I'd think that the regular rear Ortliebs would hold even a 17" laptop. Weight would no issue for those bags. You could buy an "sleeve" (a padded envelope bag) and use that inside the pannier. You could probably even fit a regular laptop bag for large laptops inside the regular rear Ortlieb pannier.

Mechanical hard drives park their heads when shut down, so it's not really a big deal to have them in a bag unless you're planning some radical riding. However I'd highly recommend the SSD just for the performance advantages---they're a huge improvement.