Looking at spending the banks money on a power meter and stages/4iii 105 left hand crank seems the cheapest and easiest option to add.
I have a Shimano ht2 preload tool already and all the allen keys in the world too, but I was wondering if since it measures the strain the pinch bolts should be done up using a torque wrench?
If so how accurate of a wrench should it be? The basic deflection style topeak combination one or an expensive clicky park one?
No, you don’t need a torque wrench.
I use torque wrench for my 4iiii solely as I have one. Certainly wouldn’t buy one especially. Just do up ‘tight’ but not silly and you’ll be fine
No torque wrench required. Picked up a Stages left side PM recently and couldn’t be happier with it. Just bolt it on and go.
Same here, just stick it on as per the one you’re replacing, it’s a doddle and off you go.
Ultegra 6800 or 8000 crankset just pop the left arm off, put the Stages on, and good to go. Exactly the same with 4iiii.
I like torque wrenches, use them all the time on my bike. I think they are especially important when working with carbon.
Pedal bolts are roughly 12-14NM IIRC, I’ll guess that most people who say they don’t need a torque wrench do them up to about 30! 
doh! Yup, I use torque wrench on the pedals and have a carbon bike and its best practice to use it everywhere else.
I have found a torque wrench to be invaluable, especially as I began making more adjustments/repairs on my own. There are some good, not spendy wrenches check out amazon or maybe look at Performance Bike’s closing sale.