I have not heard of RAM compatibility issues other than a 2GB maximum. As long as the RAM you buy meets the minimum specs and is from an established vendor, it should be fine.
As for SSDs, there aren't a lot of choices here. You need to make sure that the one you buy is designed to fit and work in the Mini 9. If it's for the Asus Eee, it won't work. There are several brands available, but only the RunCore models outperform the stock SSD
Instructions for replacing the RAM and SSD
are provided in the Dell documentation. I would recommend changing one thing at a time to make sure the new components work.