If this will be powered by your MidMonster, then the 2 speed solves all the dual use (big road top speed and quite slow steep climbs with a big load) worries I had in terms of gearing. Just be sure the low gear goes fast enough for your most used riding within a few klicks of the house, and high gear isn't geared for an unattainable top speed for use on the big road. Something around a 2:1 gear ratio or slightly less should accomplish that. If you need to switch to high frequently, then you'll end up too steeply geared for much of your more local climbs.
The 2 speed also makes it less important to push to as high a voltage as possible. I'm running my stock MidMonster at somewhere around 15kw peak input with a 30s pack, but I'm only able to go that high because my small wheel puts me geared just right for my roads and load. If I ride foolishly with too many full turbo takeoffs, then I could heat damage my system, so I save that for the opportunities to teach motorcycles a lesson in electrics. 28s and a pair of the max 70A controllers I mentioned is perfect and economical for your needs. Gear for a no-load speed of 95-110kph in high and 55-70kph in low, I think would be the optimum gearing for your needs...maybe a bit slower for low gear if you want less than a 30mph top speed for local riding.