Thanks for the answer!

It does get great gas mileage, which is something I love about the car. It's actually a bit complicated as far as the repairs go. The spark plug failure was caused by a bad valve that let oil spew out and gunk up the plugs (I was told that this was a possibility by another mechanic previously as well - apparently it's pretty common in my car), and the tensioner needs to be replaced along with the timing belt. From what I was able to research online, $1000 seems to be a reasonable price for the repairs. The shop is also pretty highly recommended for not price-gouging.

I do think that considering this is the first major problem I've had with the vehicle since I purchased it two years ago, I wouldn't bet on another major problem in the near future, but as you said, I don't have a crystal ball.

I just don't want to get into that sunk cost mindset, and keep spending money on problems as they crop up.