Flux is clearly correct on all accounts.  

I would not even consider a centrifugal pump for 170 meters of head.  Positive displacement only makes sense.

Also correct is the problem of directly connecting a 4 meter wind turbine to a 5-7.5HP pump.  (Side question, did you check to be sure 5.5KW is big enough?)
My limited understanding of wind turbines shows that 4 meters is too small for that size pump.  Unless you can increase wind turbine size, I agree with Flux that connecting 2+ turbines to syncronize AC outputs is doubtful.

"Feeding dc to a vfd seems simple in theory but you would need a fair bit of circuitry to hold the volts within limits. You will also need a supply for the drive control circuits."

This is spot on.  The circuitry to regulate the DC Bus voltage is tricky but doable.  The key would be its speed of response.

"It is likely that you will be forced to use a standard unit and modify it. Otherwise starting from scratch I think a vfd could be produced to accept a dc rail that rises with the motor speed just as the motor will accept raw ac as long as it varies in voltage and frequency in the correct ratio ( which it does in this case)."

15 years ago we had VFD's that had a DC Bus voltage which varied in amplitude and we regulated the output frequency to the motor based off this voltage....sadly I filled dumpsters with many of them.  I haven't seen one in 8 years.  They were called PAM (Pulse Amplitude Modulated) drives verses todays PWM's.

My opinion, but if you want to stay simple, and electric, then FLUX's suggestion is the perfect solution "Seriously worth thinking about using more smaller dc pumps"

You could rectify the output of all your wind turbines, connect them in parallel, and feed it down to a Positive Displacement DC well pump(s).  No inverters, no batteries...simple solution.
Remember the wind turbine works off the affinity laws just like the pump does...sounds like a perfect match.

I also agree with Oztules & Dinges posts, water pumper mills are proven, and easier to maintain by most people.

I think the DC pump system would also be easy to maintain.