I'm a little confused. Why not "amerikansk"? I remember seeing somewhere that -ska often pertained to a language (i.e. svenska) whereas -sk was for other things related to a country (i.e. svensk). Is it different here because the question contains a plural?
Yes, the "...som helst" construction works better for standalone things, like "vilken bil som helst" for "any car". For expressing the "ever" in uses like "whatever you do", "whichever car you choose" etc, "vilken ... än" is used.