Bare minimum you need a mic (with quarter-inch plug) and headphones. If you wanna play it over a speaker so you don't sound like a gibbering tool to any roomies/neighbours, you'll have to add speakers (any type, recommend something powered so you don't have to bother with a seperate preamp) and the right cable to hook up to them. The KP3 uses what's called "RCA" or "component" cables...find out what end you need to plug into your speakers and make sure the ends fit, your local music store should have something suitable. I say "speakers" because it's best to have stereo with most of the FX on the KP3.
Young people perhaps - that was my impression. Plenty of 14 yr old Dubstepping beatboxers around. Everbody has to start somewhere.
To the OP: Reading the manual before asking any more questions would be a respectful idea though. Best to ask for help when you are truly stuck & are not gaining understanding from the printed word.
I actually don't have a problem with it at all -- it just surprises me! Personally, if I was saving up my money to spend on a piece of audio gear, I'd be watching YouTube videos and reading reviews to make sure I understood what I was buying.
You can shop however you want! I'm just saying, your question was about the equivalent of "I just bought some drums, do I need sticks or something?"
When I bought my new set of gear, I spent about 4 months figuring out what I exactly wanted, and what I could do with it, how much money I needed, what kind of cables I needed, etc. When I was doing a summer job, which was archiving documents into a computer system, I was constantly listening at demo videos and stuff.
So yeah, I agree. Who the hell are these people who earn enough money to casually buy a KP3 with a decent microphone etc.