Encryption Strength

Ok, I don't know if the place for this, but I'm getting desperate, and don't know where to go.

I've got to write some kind of encryption/encoding scheme. I can't use non-printing characters in the output and there will be no non-printing characters in the input. The key will be a random generated series of 0-15 (4 bit words). The number of words will be predicated on the length of the password. Assume a minimum 8 characters. The password will actually only be used to encrypt the key.

My problem is: How do I describe the strength of such an animal?

The system doesn't have to be perfect, indeed, given the parameters, it will be relatively weak. I need to convince an ISO that this scheme will allow me to email confidential data through a semi-secure system with assurance that his next-door-neighbors kid won't crack it.

I've tried to read the literature, but I can't find anything that address this particular problem.