Thanks for all the advice! As I don't want to do anything wrong I think I will go for Geits suggestion.

I don't need a big SSD as all my stuff is stored in an external USB HDD for easy swapping. I only use some 20 GB for my MorphOS need so a 80 to 120 GB would be more than enough.

Since I'm very insecure when it comes to hardware: from which international Ebay sellers can I buy the adapter and an SSD for my needs?