A "Core" system is basically the brain. Diskless Media Directors (hooked up to TV's to play movies, etc.) are appendages.
I would suggest that you spend a little more time reading the wikis, decide if you want a core or a core/hybrid etc.
An easy thing to start with is a core/hybrid you get to see the whole thing in action.
When I started to play with the system I tried a pure Core, then wiped everything, played with a Core/Hybrid, wiped everything, reloaded the software about 12 times, at least. I thought about what I wanted to accomplish and eventually after playing for a long time, (months in my free time), I decided that for what I wanted to do required a dedicated core and MD's or susbsystens.
Read, play and have fun.