Cartridge and needle are just different parts of what most people call "the needle". But actually, "the needle" is a headshell, wires, cartridge and stylus. And the stylus is literally "the needle" that touches the record. It's like this:
The stylus is the little metal needle that you put on the record.
The cartridge is what the stylus slides into.
The headshell is the large flat piece that the cartridge is connected to by screws and 4 wires.
And the headshell is connected to the tonearm (the long S-shaped rod).
Generally, any headshell will be able to hold any cartridge. So when you start out, you'll have a headshell (Turntables often come with these), you'll buy a cartridge and stylus (they come together), but when the stylus breaks or wears out, you can replace just the stylus. They're sold individually, so it saves quite a bit of money. But stylii only work with the cartridges they're designed for, so if you want to switch brands of needle, you'll need to buy a new cartidge/stylus combo. Probably not a new headshell, though (I'm talking generally here, some cartridges won't fit every headshell, and there are even some designed for use without a headshell).
As for whether you need a direct drive, or any Turntable at all, I'd say go for it if you got the cash. I guarantee you'll find a lot more uses for it than you have in mind right now. Hell, once you start scratching, you might find yourself addicted. A lot of people just start out messing around with DJing, and then the bug bites them and they never look back.
EDIT: Forgot about installation difficulty. Headshells and stylii are braindead simple to install. They're designed for quick interchangability. However, hooking up the 4 tiny wires from the cartridge to the headshell can be tricky, but nothing big. It's more tedious than difficult, it's a really small area, and it can be hard to get your fingers in the right spot. I use tweezers a lot. Oh yeah, I should mention that optimizing your needle's angle, forward position, and height can be a chore. Once again, it's not difficult, just time consuming and boring, but it's not something you have to do every time you get on the decks if they're stationary.