As I understand it with an MEP rating you can fly any MEP aircraft without a check up to 5700Kg MTOW. Above this size you'll need a type check. However, you will also need to note that by law, with a PPL the PIC must pay all flying expenses for aircraft with more than 5 seats.
Out of interest why do you want to get the IR and IMC?
Finally you need 70 hours P1 before you can get an MEP rating though I wouldn't touch anything as big or complex as a king air with this minimum amount. It is a lot of plane to handle, especially alone.