Simone,
So, as you are bringing the system and the administration in-house from a hosted situation, do you already have a DBA on staff to help with that aspect of things? Jim Spath may proclaim to be "always a DBA" and not any kind of Basis expert, but often the reality is that a good Basis admin has to also be at least somewhat of a DBA. So, with that in mind, beyond the "pure" Basis classes and books that have been mentioned, you might consider finding something on the database platform.
Are you going to migrate to something else as you bring it in-house? DB/OS migration is another area of specialty for some, so you might still consider using a consultant to help with that, especially as it's presumably a one-time or at least rare thing to do, therefore not necessarily critical for your own skillset and experience (but you will get good at homogeneous system copies 
).
Finally, if you're now going to be the new Basis admin, who is going to replace you as the ABAP developer? Surely your employer doesn't think it a good idea for one person to do both at the same time, do they? If they do harbor such fanciful illusions, you might point out the benefits of separation of duties, i.e. do they really want the same person to write the code, release the transport request, import the transport to production, and monitor all the logs once it gets there?
On second thought, maybe you do want to keep it that way. Think of the embezzlement possibilities. 
--Matt