For xcode, a faster CPU will make an incremental improvement but unless you are working on mega projects with massive, lengthy compiles, it's probably not worth worrying about. If you're going to use an external monitor, than you can get pretty much anything with enough memory and it will do fine for you. If you are going to be limited to the laptop screen, you might think about a used 15 inch MBP just for the screen real estate, although hitting your budget might be harder. I'd also strongly suggest 8 Gb of memory, or at least more than 4.