Planning to buy a used iPad. Two questions: 1) What's the oldest iPad that can take advantage of iOS 8.1 & OS X 10.10 improvements? 2) And what functionality would i lose if i go with an iPad too old? I noticed iPad 3rd gen's are pretty cheap on CL but iPad 4th gens are still pricey and iPad Air's are still way too close to retail store prices. Wanted to make a more informed decision on what's the best value before i buy. Thanks!