Mac Mini restoration?

I have a 2010 Macbook Pro, and I love it. I just recently upgraded from 4gb of ram to 8gb. I kind of liked the whole idea of user upgrading, and it was the first time I had ever opened a computer. Anyways, my MBP is working fine, but I kind of want a really fast and powerful desktop, but I also want a computer I can upgrade. I was looking at older mac minis on ebay. Some go for $250, but have slow processors and horrible specs. How upgradeable are older mac minis (2009ish)? Can you go beyond the RAM and SSD?