Depends what you're wanting to do with it. Obviously, they are at the very bottom of the line, as far as Macs are concerned, and mid-range as far as PC's are concerned.
As Gil said, it depends on what you're going to do. If you plan on doing a lot of graphics work, then you should go up to a higher range model (such as the iMac, for example). However, if you don't plan on doing those things and just using it for basic tasks (well, you can do graphics work on the Mini, but you might find yourself limited, in terms of RAM and graphics capability), then the Mini would be the most affordable option for you.
you have to buy everything else extra (which adds up)
No you don't. That's why it's called
Bring Your Own Display, Keyboard, and Mouse, not Buy Your Own Display, Keyboard, and Mouse.