What are the advantages of a building an electric bike from a kit vs. prebuilt I'm trying to decide on what would be the best option for me for my first electric bike. I need a way to get around, and as I live on a hill where I will have to manage a fairly steep incline coming and going I am wondering what kind of bike I should get. 

There is a bike on Craigslist right now with a 36V 12amp battery and 500-750Watt motor, but as it is a Huffy bike I am pretty sure it is a kit build. They are asking 700. -- My other option is the Kickstarter kit by Barak Electric (https://www.kickstarter.com/projects/1354698863/barak-electric-bicycle-conversion-kit-electrify-yo) that has a 500 watt motor and 36V 10amph battery, and as I already have a mountain bike laying around I could install it myself and it would cost 655. I have no idea what they would be charging in a couple years when the Lithium batteries die for replacements, but I'm sure I could figure that out then. I don't have a ton of money, so going to a bike shop isn't really an option even though I would love the security of being able to return it if it doesn't work. I was looking into the Currie Electric bikes as they seem quite reasonable, but it doesn't look as though they will have enough power to get me anywhere with their small wattage and SLA batteries (not to mention to upgrade to Lithium with Currie is basically the price of the bike!) 

Any advice would be greatly appreciated!
First, check your local laws. As others have mentionend, in some countries (EU), maximum power for an electric bike is 250W. Go beyond that and the vehicle will be considered a moped and will need license, insurance and and license plates. Your local laws might also require an assist-only option for your bike to be considered an e-bike. That means usually a pedal sensor, and those can be difficult to install especially with exotic or outdated bottom brackets.

Selecting a kit is not necessarily easy. If you're going to front mounted motor, check that brake type is compatible (disc or v-brake) with the kit and that fork spacing is suitable. E-bike kit should come with brake levers with a cut-off switch so the motor stops pushing immediately when braking (problem with some e-bikes).

Kits can be had cheap, but installing them takes time, effort and experience with bike mechanics. If you got all three, go for it. If you need to get up and running quickly, an entry-level e-bike is probably better option.