Eh, there's bad history between the FalconEV people and members of this forum. Bad things were said. Not that that matters if you're getting good helpful advice.
Now, an important correction. You have your units all wrong. Battery capacity it a given voltage is measured in Amps*hours, not amps/hours. Energy capacity, which is what really determines range at a given speed, is measured in Watthours, which is Amphours*Volts. Your efficiency is in watthours/mile, where a lower number means less energy to travel one mile.
I'd also suggest a recumbent, trike, or recumbent trike. On an upright bike, that commute will make you walk pretty funny when you get to work, even with a good gel saddle. In a recumbent, your aerodynamic profile will be far better, and depending on the style, you could have much more room for batteries. The latter also applies for an upright trike. In fact, one member built an electric upright trike with 50miles of range using 6 marine lead-acid batteries. Heavy as hell, but relatively cheap!
Since speed isn't important to you, and it doesn't sound like you need to climb mountains, a crystalyte 400-series motor or similar would be a great fit. Light, powerful, and reliable. I'd also suggest you run it at 48v, which will give you much more power than a 36v setup while allowing you to avoid using more expensive 72v controllers.
To do 50 miles at 25mph, you'll need ~1400Wh of rated battery capacity, which would mean a 48v 30ah pack. That's assuming 450 watts of power to keep that speed, 80% motor efficiency, 80% depth of battery discharge (a good margin to keep the batteries healthy over the long run, and to account for other small losses), and assuming no pedaling. If you choose to pedal, you'll be able to add 150w of power (that's continuously). Doing so would reduce your required battery capacity to ~950Wh. That sits nicely with a 48v 20ah battery.
Sometimes gearing can be a problem. You'll need to gear your bike so that you can keep a comfortable pedaling cadence at 25mph. For whatever bike you choose, if you choose to pedal you will need to first ensure that you actually can keep up with the motor. A mild downhill where you can reach such a speed with a little extra pedaling would work. First see if you can get to 25mph, hold your speed there with brakes if the hill's too steep, and see if you can comfortably keep the freewheel engaged at that speed. If not, you either need taller gears, to go slower, or not pedal.
So, other than a comfortable recumbent or whatever bike you choose, I recommend a crystalyte 408 motor in a 26" wheel, a 36v 20a immediate start controller (you could use a more powerful 35a controller, but it'd add ~$100, and at that voltage and speed you'd only see a benefit on steep hills), half-twist throttle, and 48v lithium iron phosphate (LiFePO4) battery. That isn't the only way to go, but it's one of the more popular and well-tested routes. You'll have the choice of putting the motor in the front or rear wheel, so choose whichever works better for your bike setup. I highly recommend you look at the CycleAnalyst (http://ebikes.ca/drainbrain.shtml). It's the best ebike computer there is, and it'll tell you everything you need to know about your speed, distance traveled, battery status, power use, efficiency, and more. Also consider buying everything from ebikes.ca. They're the most knowledgeable and helpful sellers in the business, and they won't gouge you or sell you anything you don't need (cruise control, brake inhibits, etc). Their controllers are also modified to directly plug into the CycleAnalyst (which they invented, manufacture, and continue to update), which saves a lot of trouble.
For the battery, you have a few options. Ebikes.ca sells LiFePO4 batteries in 48v12Ah configurations, so two of them would provide 1150Wh. They're very well-made and well-documented, but they're also relatively expensive. There are also "duct tape" batteries. They're sold on Ebay by a fellow named Ping, who buys the cells and assembles them in China. They aren't as well-built as the stuff other places sell, but many here have had nothing but good experiences with them. Two (or three, if you don't pedal) of these would do: http://cgi.ebay.com/LiFePO4-Electric-Scooter-E-Bike-Li-Fe-Battery-48V-10AH_W0QQitemZ220226704324QQihZ012QQcategoryZ11332QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
Any questions? If you have 'em, we'll answer 'em!
