The early 2008 Airs are completely crippled with a painfully slow processor, 2GB of RAM and awfully slow, 1.8 inch ATA drives. The late 2008 and mid 2009 models are massive improvements (with much improved 9400m graphics and faster processors) but they still only have 2GB RAM and in general still run really, really hot.
I'm guessing, giving the level of work you're going to be doing on this machine, you're not in the market to go out and spend a metric shed-tonne. So I'd avoid the 2011 (1st gen Intel Core) models and get a high-end, late 2010 Core 2 Duo based MacBook Air with 4GB RAM. IMHO they're the best bang for buck in the used market at the moment.
As soon as you hit the 2011 models with i5 and i7's, the prices go up massively - and although the processors are quicker, the graphics are actually a massive step down. The late 2010 Airs pack the really nice 320M, which, paired with 4GB RAM flies through Yosemite really nicely. The 2011 Airs have the integrated HD3000 chipset, which all in all is a seriously weak piece of kit. After that you've got the 2012, 2013 and 2014 models which are more well-rounded packages, but are again a big step up in price, and by then you're getting to the point where you may as well go to Apple directly and pick up a new one.
Maybe I'm biased, but I picked my 2.13GHz late-2010 Air, with 4GB of RAM and a nice 256GB SSD up for a decent price, and I carry out similar tasks to what you're looking to do - web browsing, web design, photo editing and a little bit of editing in FCPX and it handles it all like a champ. You get full support from Apple, along with the awesome design that the new Airs still pack today, all for a fraction of the price of a Core series Air.
TL;DR - Avoid 2008 and 2009 Airs, get a 2010 Air over a 2011 Air. If your budget can stretch and you need an i5/i7, pick up a 2012 model. And whatever you do, make sure it has 4GB RAM minimum.