What is the oldest MacBook you would buy in 2015?

Im starting my senior year of high school in a month and I want a MacBook to get through it. I have about $400-$450 to spend, I plan on buying a used or refurbished one from a trusted seller on eBay. My question is what is the oldest model of any MacBook that is okay to buy in 2015? I need it to run the latest software for the next year (maybe two). I will mainly use it for web browsing and school work, something I can carry around at school all day. I want it to be fast and run OS X Yosemite smoothly. Flash storage is not an issue, I do most of my school work on a jump drive or Google drive.

Would a 2009-2010 13 inch polycarbonate MacBook be a good choice in 2015? I want a newish 11.6 inch MBA but I don't know if I can get that in my price range. Do I need to look for something with at least an intel i5 or is an Intel core 2 duo okay? Do I need to look for 2 or 4 or 8 GB of ram? I just don't know what to look for.