I read various books/texts, Sheldon Brown, Roger Mussons, Jobst Brandt, Gerd Schraner.. the list goes on. Most make reference to the listening technique for identifying even spoke tension. However, I had two issues with doing this. Firstly, it does not define the correct spoke tension - it just assumes you have the right tension and after making them sound the same, they are all even - but it could be either too high or too low.... Most seem to have an equally rule of thumb means of deciding that they have the 'correct' tension which was insufficient for me.
The second issue was that the wheel I built with the listening technique sounded equal and ensured they sounded similar to bike with the same hubs, rim and consequent spoke length. The spokes didn't ping or do anything unusual. Within a couple of weeks, the spokes pulled though the rim, splitting the rim down the middle and allowing the tube to poke out! Made for pretty entertaining ride home but definitely not something I'll be doing again.
The conclusion drawn from this was: I had all the tensions relatively too high and either the listening technique requires development of skilled hearing over time.. not suitable for someone doing it the first time round. I've relied on the park tool measurement and I have built a number of wheels which have been subject to a decent amount of use/abuse. No issues since!
Is there a spoke tension meter that would fit (probably can't be wider than about 60mm or so) and not cost too much (ie < $100)?
The only tension style meters I've seen since which are thinner (meaning the distance between the points which touch/bend the spoke) were textile or wire tension meters. I'm not sure if it will apply well as the ones I've seen would force the spoke to move at more aggressive angles and I'm not sure if they would cope with that higher tension. I imagine google is your friend here.
Can such short spokes withstand being dished 5mm without over stressing them?
Its hard to answer that definitively because its a function of the spoke seat angle as to how far they can be angled without bending. Grin actually has a decent explanation and points out the max angles on their calculator from memory in relation to choosing spoke length and I think the same concept applies here. The dishing should be coming from the position of the spoke nipples and spoke head direction, not bending or stressing the spoke. I've gotten away with bending before, but really, faced with this issue again I would be looking to get a rim with the correct spoke hole angle or potentially filing it a little to allow it to correct it more than relying totally on bending spokes. The better option is to choose rim/spoke/motor combos that steer clear of these stresses leading to avoiding premature wheel part failures.
Is there anything else I should be concerned about or be considering?
I have been blown away with the power to weight ratio difference between DD and geared motors and I wonder if it would be worth considering a lower power geared motor which also has a smaller dimension - allowing you to get away with longer spokes. Bafang BPM motor has historically been known to put up with a fair amount of abuse... food for thought anyway.