Most critical for your design will be , back-EMF quality (harmonics,imbalances) and amplifier quality (offsets, imbalances). Of course sine commutation and core-less type of PMSM will be preferred. As a rule not need calibration of digital encoder.
I hope that you have possibility to measure gain/phase of position from encoder vs. supplied sine as well. The slope of Bode for such case must be (-40dB/decade). And any resonnace will be reflected as pair of anti-resonance and resonance frequencies.
No - you must use a controller for BLDC especially firstly for commutation.
And BLDC pump + a controller would be expensive cheaper than a normal AC pump + a inverter as well.
The wreewheeling (flyback) diode must be strongly recommended for activation an inductive load from PLC Output - you can find such recommendation in PLC manual as well.
Except speed 10m/s, you need calculate acceleration/deceleration also. And ones maybe very high also - your speed profile even can't reach such speed! It seems that the only possible solution will be linear motor based actuator.
One of key desadvantages of suggested above "smart relay" is inability to control such DC motor with 15A current. Than single axis motion controller with build-in power amplifier can be better way - for example from Elmo, Copley, AMC.
You must thinking about current limitation with direct connecting to 12VDC power supply as well. But with using PWM power amplifier current consumption maybe lower than calculated above.
