This is what the code says:
126.96.36.199 Staff conducting internal audits shall be trained and competent in internal audit procedures.
188.8.131.52 Where practical staff conducting internal audits shall be independent of the function being audited.
Did you undergo an internal auditor training? It's not required but it can help you.
1. You need to have a cross-functional team that can carry out internal audits. You and other members, of course train them. Desk auditing part, For example you will need someone else to audit the paperwork (SOPs) you formed (have them follow the code and check out everything). Facility audit part, Production manager for example to audit the warehouse and not production part. Warehouse manager to audit production. like 184.108.40.206 says they shall be independent of the function being audited. You can audit every area in the facility, with your team unless its your function.
2. I don't see the need for bringing an outside company... The code says "staff" .Just have you and other staff like I mentioned before.
3. As long as she is competent, trained in IA procedures, I don't see why not. She can be part of your IA audit team if you go ahead and include her in it. But what do you mean that you own another company that "is not related" ?
Make sure there is someone scheduling the audit, writing the reports, following up on the CAs and closing them out and ofcourse documenting them and maintaining the records! Make sure it is reviewed and validated annually during Management review meetings.
In my company there is me (SQFP), Operations Manager, Production Manager & Maintenance manager as part of the IA team.
I hope that helps.