As long as it does not get a stuxnet virus and secretly kill them tomatoes mate 
I will go ahead and copy and paste the message I received from a highly skilled electrical tradesmen I talked to.
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The type of PWM motor control I'm familiar with is on treadmills. They use 12V DC motors up to 1 horsepower, maybe more. I find them on kijiji or craigslist occasionally for free. I've used one to make a variable speed belt sander, and I've seen them used for drill presses to get the speed down low enough for drilling metal.
I thought I had let the magic smoke out once when sanding metal, and when I looked online the control was about $200.00. Luckily I was able to blow the dust off and the controller still worked. I then built an enclosure to keep the metal filings out.
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My pumps max draw is 15.5 amps in worst case scenarios according to the spec sheet. In my use case the electrical load is lower. I think I'll be ok with any ebay fare rated over 20 amps at 12v. Finding a ok priced tredmill and getting it home would be a daunting task for me and the one i end up with may not have 12v motors
I ended up with this Controller.
There are nicer ones with an enclosed waterproof housing available for a similar price or less if you order from china but I did not want to wait that long. This was the cheapest available from a north American seller.
It works fine. I now presumably am using less electricity and getting the same work and also its much easier to dial in the best possible flow rate. At low outputs the pump dose shake bit more and is much quieter. I don't think its putting a strain on the pump using it like this but I suppose time will tell
Not sure if anybody is still interested but I think Ive gotten to my final configuration.
I now have my motor controller mounted on top of my battery. The only electrical problem I had was since this is DC at higher current and electrolysis is a thing any exposed negative terminal or connector would quickly corrode. I solved that by applying Dielectric Grease to any exposed terminal or connector.
I now have two lines with valves going into the pump, one goes into my well the other goes into my 50 gallon barrel. I can turn off the well connection and pump water out of my container for whatever I want now and if I open the well connection with the container also open it quickly self primes then I just close the container side to pump the well. I can fill my container in about two hours and that is usually enough water to last me a week.
edit: Cats are oddly attracted to my pump when its running. I get several strange cats a day. Some times they try to rub up against my hose and pull it lose. They seem to think its purring and try to make friends with it?? its weird
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I added a few things to this system for anybody interested. Now it is actually solar powered. The actual watering is done with an electronic timer and a gravity fed irrigation system consisting of drip tape for my rows and some drip emmiters for my raised bed. I still fill my two 55 gallon drums manually and I come out to furtalise but day to day is automatic. The drums are up on cinder blocks so there is enough pressure to feed my irrigation system, the only thing the pump dose is fill the tanks. Most of the pump system is the same except I replaced the motor controller with a better quality one with a higher amp rating and a waterproof houseing. Solar panels with a basic solar charge controller now charge my deep cycle battery. For whatever reason several cats still show up whenever the pump runs.
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Could you post pictures?
I am very interested!
Well to be honest I kinda likes my privacy and I don’t really wanna risk mixing my online and real life. This is an outdoor setup people I know see, I do not know if they happen to frequent the forums.