Automatically Assign Predefined Unit # In Separate Worksheet

I have a worksheet that logs equipment activities. Equipment may already have a standard unit # assigned to it or it may need a temporary unit # assigned for tracking. In a separate worksheet is a column of predefined temp unit #'s that can be assigned and next it is a column to place an "X" once a unit # has been assigned.

I need to write logic that, when an "X" is entered in Column F (Assign a Unit #) of the Activities worksheet, the Unit # (Column G) will automatically select the next available Temp Unit # in the Assign worksheet...and mark that Unit # as no longer available. Quite frankly, this may be more easily done with VBA, but I am not experienced in that at all. I have attached a sample worksheet showing the format of the unit # to be assigned, etc.