Quote1) Can i use a regular PC Monitor's on it ?
Most games boot into 31kHz, VGA-friendly. The few games that do not can be changed over, with only a few exceptions. Atomic Robo Kid is only 15kHz.
Quote2) Can i use a regular PC Keyboard's on it ?
No.
Quote3) What must i do, to play a Game ?
Insert disk, turn on system, wait. That's it. It's not a Commodore64. =)
Quote4) What are known Problems of the X68k System ?
This is a big question. The three biggest problems are:
i. dead PSUs. There's a repair guide on GameSX.com (http://nfggames.com/nfg/x68repair/)
ii. Bad floppy drives. There is no fix.
iii. Corrupt saveRAM (or some other reason you'll get the bad error message (http://nfggames.com/forum/index.php?showtopic=2364&st=0))
Quote5) Are the Disk's still working ?
The disks never die. I've never received a game that had bad floppies.
Quote6) How many Games exist ?
Lots. As for must-haves, well, that depends on you. I put up a gallery (http://nfggames.com/games/x68k_Gallery/) of some good ones.