Python 2, 123 122 118 114 98 Bytes

Man, I wish random wasn't so expensive (and that we didn't have to filter for spaces). Now we have big savings from being allowed to have cipher characters at the end :) Anyways, here ya go:

from random import*
f=lambda s,n:s and(' '<s[0])*eval('s[0]'+'+chr(randint(32,126))'*n)+f(s[1:],n)