Skimming the responses I have yet to see a recomendation for a kit that is likely to be capable of your above 10% grade hills. Perhaps the pie can do it, but I'm not sure. Cellman sells a good hill motor, but it wasn't specificly mentioned. You'll need more power than the tiny gearmotors for those hills, and CM's direct drive motor is not the best winding for hills.
Simplify your life by getting a 26" bike, or send whatever you get to Holmes Hobby, to get it a quality lacing to 29"
For those hills, actually 20" would work best. Ever considered a tadpole trike?
We know the Mac 12t motor kit can do it. Or the 10t may be good enough, if 12% is really the max grade. Get it here. http://www.emissions-free.com/
Get your battery from Cellman in any case, 48v triangle type. 11 ah is enough if you can charge at work. If not,then the 20 ah pouch pack CM makes is good. Ask about it when you talk to CM.
The other motor I know from personal experience will climb your hills no problem is a slower winding 9 continent. Get it here. http://www.methtek.com/2011/11/12/2810-26-rear-37/
You can get it in a reliable kit, that link just shows the specific motor I'm talking about.
This motor is very slow, so you'd only have 20 mph at 48v. Many who have bought this motor are running it at 72v, and are very happy.
Riding just 8miles, you don't have to have such high speed. But your reliability issue is going to be covered a lot better by using a slower motor, like the 12t , or 10t mac or the 2810 9c. These slower motors will tolerate crawling up the steep hills better, and won't melt down on 15% near as likely as a typical bike kits motor will.
Grin cyclery sells a 2808 winding 9 continent motor, that I would definitely recomend if your hills were 10% grade max. Length of the hills matters too, if it's just 15% for a few yards, just jump on the pedals hard to get up it.
The other approach, would be to take a typical winding, put it into 20" rim, heave power at it, and climb the hill very fast. Works great, but if you care about it, the bike is illegally fast on road in every US state using the 5 hp approach.
Where are you at anyway? what continent? I've been mentioning some vendors based in north america, Cellman is in china.
THE LIPO RULES. NEVER ABOVE 4.3V NEVER BELOW 2.7V DON'T PUNCTURE
Ideal charging /discharging range for Lipo, 3.65v minimum 4.1v maximum
See battery technology section, FAQ thread at the top of the page for lipo noob info.