MacBook Pro 2011 SSD/RAM compatibilities and investments
Answer to question 1-3:
1.SSD or RAM?

My suggestion is 512GB SSD and a memory upgrade.

2.5inch SSD that is used in an old MacBook cannot be used in a new MacBook. So 1TB is not necessary.

512GB SSD and RAM upgrade can ensure performance and avoid the choice between SSD or RAM.

2.RAM selection?

Since you already have a 4GB ram, adding another 8GB ram could work just fine. It is ok if you want 8GB * 2 as well.

RAM frequency could be 1333MHz or 1600MHz, no need to consider about compatibility.