Your client should be concerned about hiring someone who doesn't know of any affiliate scripts to use, and hasn't discovered the power of typing in:
"I need an affiliate script" into google.
Now that I got that out of my system - the answer depends totally on how the client is set up, if he already has a website, if he is already making sales, if he is drop shipping, if he has to deal with xyz, if you have to integrate into an existing sales system, etc...
Check out jrox if he already has a site - it should integrate with any network out there or be easy enough to integrate. Because you aren't going to get to 1000k affiliates with out a lot of marketing. Just cuz you build it don't mean they will come.