My experience suggests you should only internalize functions that require on-site employees. I have used external bookkeeping resources for more than 10 years and been very happy not having that function in-house. Obviously there are caveats depending on what you mean and how you are setup. Sometimes it makes sense to have AR/AP internal and sometimes it doesn't, however in my 10+ startups I have always outsourced it and always been happy with the results.
Office Manager and Admin can also be different things. Virtual Assistants can easily pick up the slack on admin tasks that you would otherwise pay too much to have done in house. Office Manager may have to be onsite, but you really need to consider what you can profitably outsource.
Happy to discuss my experience and make recommendations based on your specifics as there is no one-size-fits-all answer for personnel. There are many variables.
