I had a 2011 13" i5/4gb/256gb MBA and upgraded to a 2013 11" i7/8gb/512gb MBA. Very happy with the new machine. I was running out of space with the 256gb SSD, but I sometimes edit video and those files are huge. But I like to keep as much as possible on the internal disk and only use external drives for backup and archives.
However, what you describe doesn't sound like you really need a fast CPU, a lot of RAM or a big SSD. If budget is important, I'll bet the i5/4gb/256gb model would be more than adequate (personally I am running powerful software like final cut pro, logic pro, filemaker pro, vectorworks, etc). Future proofing always seemed like a questionable concept to me. A few years from now, you are just going to have an old computer that is slower and has less memory/disk than the new models anyway. I would only buy what you need now, or what you will need during the next year or two if you plan to expand into more powerful software.
Of course, if you can afford it, the i7/8gb/512gb is very nice and the extra speed, RAM and storage is certainly not a bad thing. 🙂