You have a very short round trip, so a lead acid battery should be more than fine for you, but these, although cheap, are not as available as they used to be 10 years ago. If there is a hill, then you only need the battery one way.. (and it can suck up power on the uphill).
The conversion kit with a front wheet replacement is the easiest, with the battery sitting on a rear rack. You won't be jumping over logs or curbs with the front wheel kit, but otherwise they work great and are the cheapest options.
Getting an ideal fit of a bike is less important when you have the motor on. That last 1" adjustment really is not noticed.