It sounds like you are being set up for failure.
I don't know your relationship with the Client but, if possible, it may be best for both of you if you walk away now. Whatever you provide in 3-5 days will be sub-standard and you will always be the one who delivered it, regardless of the unreasonable conditions you were placed under.
That many data elements will have very intricate dependancies and relationships. Forget about even attempting to tackle any of the constraints and stored procedures that will be part of the solution.
If the Client is looking at retaining the current data your noose becomes tighter.
Add access/security on top of all that and you are sure to come up short.
Believe me when I say that you want all the relationships defined in concert with your schema design or you will be creating tables for nothing and creating some truly horrendous workarounds later to make it work later as new relationships are discovered.
