I was in your delimma when i got my mustang, and still sorta am. I am also in college and money is tight.
a sub wont make your sound system better, it will just make your base hit harder, but everything else will still sound like crap.
I installed 4 alpine type s speakers ( 2 in the front doors, and 2 in the rear deck) and it made a world of a difference. I got my alpines for 150 bc alpine was running a buy 1 get one 50% off special. Also the speakers are pretty straightforward on the install and there are many videos giving you step by step instructions on how to do it. If your just a little bit handy you can do the install
In February I am probably buying the pdx -f4 amp from alpine Alpine Electronics of America, Inc.
which is pricey, but a very good amp. This install should be a little more complicated, but i plan on figuring it out as i go.
Obviously you dont have to get the amp i am, but this is what my speaker guy recommended. you can always do the speakers now and then do the amp later, that is what i did.
hope this helps