Timing belts is #1 because of the major repair that almost always results when one of those components goes South. IMO it's shortsighted to do just the belt if you're DIY, so the full job also includes 2 pullys and the water pump. The '04 in particular had a run of early failures, so it's highly unlikely that an '04 would not have had this job done, one way or the other.
I recently replace the muffler on my '05, due to rusting of the pipe connecting to the mid pipe. The entire system is rusting, so I'm expecting other pieces to go as well. Take a look at the rear driver-side wheel well at the muffler/mid-pipe connector, to get an idea how yours are doing.
A couple weeks ago one rear shock failed, so I replaced both of those - front struts to be done soon, but those are just normal wear items. Oil pressure sensors seem to be common items to fail, but I haven't seen that yet on either of mine.
Search the archives here because there's lots of good prior information, much of it from folks who are no longer active on the forum.
You might have to quit your job in order to be able to maintain 4 older Aveos - good luck with it!