Welcome to the site.

The Aveo is a fine vehicle as long as you keep up with regular maintenance. Its when you completely ignore/abuse it that it starts to become an issue. Sadly, being such a low priced vehicle this is a common occurance by many owners. Thats how I nabbed mine super cheap from a girl who let the timing belt snap!

Yeah, filling the tires up (a bit higher than the factory spec helps more) is a good thing for mileage. I run mine at max sidewall pressure on the tire. The ultra gauge will definitely be a good addition.

I wouldn't bother with the K&N unless you're looking for a little more power. Filters are sized for peak engine power, and unless you're really mashing the pedal a lot its not going to be a restriction at lower power outputs (thus the stock one isn't hurting mileage).