Short answer...
1. yes
2. yes
Yes you can upgrade your HDD to a larger one, or use a USB external drive.
I dont know if you have a space for a 2nd HDD as I cannot access the manual at the moment, but there is a solution either way.
If my memory serves me correctly, I think there is another space for a 2nd internal HDD. If it does, there might not be a bracket for it. You would have to buy that separate.
Technically yes you can change the video card to a 1080ti.
But you might also need to change the Power Supply.
Also you would have to measure the space available where the card sits, and compare that to the length of what ever card you choose to buy, to make sure it will fit.