Hello,
1) You must grab the opportunity as it is a great opportunity in term of carrier growth and job stability . As the client company does not have any ' No Hire' policy with the employer they should not have a problem to proceed with hiring you.
2) The fact is that you are not bound by conditions laid out by the company once you resign. Follow exit policy and resign by serving proper notice to the company.
3) No company or policy can stop you from joining any company once you have complied with the exit policy as it is your constitutional right. At the time of resignation, let the company not know that you have any intentions to join the client company.