The centre bore on a k11 wheel is smaller than that of the k12.
oh, congrats on the purchase by the way 
You have the "keyless entry"? My mum lost her cap/cover thing and ordered one from Nissan, I think it was about £5
a standard (but full size) steel wheel may fit where the space saver currently is, but an alloy will stick out as they are usually wider/fatter
