The following estimates should answer the questions:

Lateral hydraulic load: 59 N
Shaft bending load: 132 N m
Torsional load: 46 N m

Shaft diameter estimate: 31 mm
Shaft natural frequency: between 100 and 200 rpm
If used with a variable speed drive, do not allow the mixer to operate near the natural frequency.
These estimates are based on the limited information supplied in the question and should be reviewed by an engineer when more complete detailed information about the mixer is available.