Hi Florian,
Depending on how toxic the drugs you are working with (and how your safety department would want to deal with that), you may want to consider the implications of various strategies.
If you buy the HEPA unit and place the system on the bench, you are going to be protecting your samples from contamination, but not protecting the operator in that case (air being pushed out to operator in a positive airflow direction. If the drugs are very low concentration or are not considered toxic, then this may be less of an issue.
If the drugs you are working with are toxic, then you may want to take a look at this thread where the use of formaldehyde and protection from its fumes was discussed in greater detail.
Most places you can use a “minute” or “small” amount of toxic chemicals in a Biosafety cabinet, but remember that while this doesn’t push air out to the operator, the Biosafety cabinet isn’t really filtering any toxic chemical; it will recirculate in the hood and eventually exhaust out to a vent somewhere (hopefully filtered appropriately somewhere else in the house HVAC system). If you currently do this work manually in your Biosafety Cabinet and your safety team has approved that, then this might be your best bet.
Removing the enclosure as @mrthorne recommends is a good idea to help with airflow. But depending on the Biosafety cabinet you have, you will need to verify that it can maintain a proper airflow once its placed in there (make sure that airflow coming into the Biosafety cabinet in the back is not blocked too much).
There are other ways to deal with this (enclosures with negative airflow and carbon filters), but that can quickly become a very expensive and complicated endeavor. The thread here previously focused on contamination protection, so I wanted to ensure that you are thinking about operator safety too. Hope this helps!