This question has some inconsistencies, which could mean that the process is either continuous or batch.  Fortunately, the answer to either batch or continuous is probably similar.  However, other information is missing, such as the solubility of the organic acid and the particle size and density of the acid if it is a solid.  The rate of dissolution of the acid will depend more on the solubility and particle size than the mixing intensity.  If the organic acid is a solid, it must be suspended off the bottom of the tank to assure effective dissolution.
 
First, all of the volumes and rates need to be put into the same units, for comparison.  Using kilograms as the unit of measure, 5,000 liters is approximately 5,000 kg.  A feed rate of 300 kg/hr might need a volume of about 3,000 kg for a residence time ten times the flow rate.  A residence time of ten times the flow rate should provide adequate mixing and dissolving for many soluble solids, so 5,000 kg, might be a good size for the stirred tank.  If the concentration of acid is 50 kg/1000kg of liquid, the the addition rate would be 15 kg/hr.
 
A 2.0 meter tank with a 2.0 meter straight side will have about a 5,000 kg capacity.  Based on the limited information about the solubility and settling rate of the acid, a 2.2 kW mixer running at 2.1 rps (125 rpm) with a 0.75 meter diameter pitched-blade impeller should be adequate.  The best company to supply a mixer is one that specializes in the manufacture and sale of mixers.  Only a few general fabrication companies know enough about mixer design and manufacture to provide a durable and effective mixer.

The answers by this expert are based on the best available interpretation of the information provided.  The consequences of the application of this information are the responsibility of the user.  If clarification is needed, please submit a further question.