Should the contract with an audit company be concluded after signing the grant contract?

According to Guidelines for Grant Applicants, 2.5.12

Yes. The beneficiary has to notify the JMA of the auditor(s) who will verify the expenditure of the Beneficiary/partners and the JMA has to approve the auditor(s) either before conclusion of the Grant Contract or before the Beneficiary’s request for the initial pre-financing instalment to the Contracting Authority. The auditor (-s) shall be selected using public procurement procedure, in compliance with Annex IV of the PRAG and applicable national legal acts.

Procurement of auditor services shall comply with the same rules as procurement of other goods and services. In particular, according to the section 2.1.1.4 of the Guidelines for Grant Applicants which contains requirements for eligibility of costs, procurement procedures can be initiated before the start of implementation period of the action, but procurement contracts have to be signed after the start of implementation period of the action.