I know its legal to build or finish a 30% lower. BUT would it be legal for me to take it to a local machine shop and pay them to do the work?

Assuming you could find a machine shop to do it... it would be illegal since once it becomes more than %80 done and they are in possession of it and they transfer it back to you, by ATF rules they become a manufacturer.

I've never built from a %30 casting or %80 for that matter, but from what I understand once the firearm is complete, it's impossible to even find a gunsmith willing to work on it if there's ever a problem with it.