Something a lady in a class I took learned the hardway... not all bobbins are interchangable. Her machine used plastic bobbins, and she got a metal one the same size and used it. It totally stuffed her machine, something to do with creating a magnetic field and it messed up the electrics in her machine.
I think she had a Pfaff from memory.
The moral of the story... when you spend a lot on a machine, it's crazy to then try and cut corner buying extras for it.