Well the 2.0L is great for fuel economy (although the 2.3L gets good mileage as well if driven conservatively). The new (I believe 2008.5 and above) Mazda3s come with all the airbag options (dual front, side-impact, third-row coverage, front seat, etc.) regardless of trim... so you should be ok.
Well since you're buying used it will depend on what's available, but each engine has advantages and disadvantages. I wanted a hatch so had no choice but to get the 2.3. If you're getting an automatic the difference in fuel economy is less between them in 07 since the 2.0 has a 4spd and the 2.3 a 5spd. Also in 07 they added stuff like the aux input, TPMS, traction control on some models.
My 3 is also my first car, and a great one at that. You want something that is safe, reliable, and has enough power to get out of its own way, but not enough to get you in trouble. Good looks don't hurt
As for the 2.0 vs 2.3, go with whichever you can afford. Both are very good engines, and if you're starting out, you probably don't want or need the extra power while you are still getting used to driving on the street.
Also, I would recommend getting a manual transmission. I learned how to drive and learned how to drive stick at the same time; it made me a much better driver because the MT forces you to focus on the road and on your car rather than fiddling with the stereo or your cell phone. Also, you will become much more intimate with your car. The first three months are hard, though, but as you pull out of them, you realize what a great choice you made. Plus, I bet none of your friends will be able to drive your car.
Ninja edit: PAY FOR SAFETY! Make sure the 3 you get has the optional airbags/ABS, and if you manage to miraculously get an 07 model or later, you could snag traction control and dynamic stability control.
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