Taking all of this into consideration, you can make an informed decision about what your needs really are in a battery, controller, and motor. Let's go through deciding on a bike or motor kit for commuting.
If you are going to convert a regular bicycle into an ebike, I suggest, for commuting rides, that you consider a more traditional frame design with the horizontal top tube, vertical seat tube, and straight down tube completing the triangle. Most bike racks on cars and buses are designed around this standard, and your battery mounting capability is maximized. This pretty much eliminates rear suspension bikes.
Your first consideration about the "e" part of the bicycle should be the law where you are going to ride it. Let's say you live in a state that just follows the Federal laws
Federal laws . Paraphrasing -Less than 750 watt(1 HP) motor, and no more than 20mph without pedaling. Most often, meeting this requirement is pre-programmed into the controller's default. It limits the wattage supplied to the motor to 750 and limits the RPM to what will drive a specific wheel size to 20mph. Many systems have an "off road" setting as well, and some systems have multiple settings that enhance either performance (top speed and/or acceleration) or economy(distance capability). A motor that can handle 750w to 1000w would be your choice here, just in case you decide to take her off road a bit. But it is better to ask before you buy.
The next consideration will be your battery. I am a fan of keeping the center of gravity low on a bicycle, so I prefer my battery to mount on the down tube. This allows the battery to be attached securely using the water bottle mounting screws and keeps it protected inside the frame. For commuting, you do not need to go crazy with the battery's voltage or energy density and this is great since the battery is often the most expensive part of the system. The battery packs we use are almost always made up of 3.7v "18650" cells wired in various configurations. There are a lot of different 18650 batteries with varying features. The latest greatest are 3.5Ah 10 amp batteries that allow the production of smaller battery packs without sacrificing energy density or the ability to draw a lot of current at once.The "Shark" or the bottle battery it the way to go. A 36V pack with 40 cells in a "10S4P" configuration will provide 14Ah. This will yield a 504Wh battery. Couple that with a 30 amp controller and you have 1080W peak. The 750 watt setting on the controller would need to be at 20 amps and you have 40 minutes at peak watts. If you pedal at this "legal" setting, you will easily cover 20 miles at top speed.