To quote from another thread: "As stated by folks like Lennard Zinn, 'Bicycling' magazine, RoadBikeRider, etc., aluminum fatigues and they recommend that these frames have a definite lifetime. The recommended life of aluminum frames is suggested to be 4-6 years, depending on mileage, type of riding, etc. Steel frames have no such limit."
As much as I value Zinn's opinion, this is just to general of a statement. Bicycle magazine has about as much authority and credibility as Bush's white house. I wouldn't worry about aluminum failing.
Have you considered a vintage bike? It can be had for ~$500. If those things are tuned properly, and the components are in decent shape, they work better than most modern bikes.