1999 Audi A4 1.8T Quattro Sedan Questions

I have a 1999 audi a4 1.8 quattro sedan and the sound system is horrible because when I connect my Ipod to it through the casette or play anything off the radio it is hard to have it loud because the bass or treble is so strong it blasts it when it goes just a little loud even though i have the settings for -2 bass and -2 treble. I was looking at the Pioneer DEH-2500UI for the receiver and I havent looked much at the other things I will need in fact I dont know all the things that you have to buy for a completely new sound system (receiver, speakers, subwoofers, bass, ) how much is all this going to cost and how much is it to install. Im 15 I can't drive yet but we have an extra car because it was my sisters old one that I will be using when I can drive. I also want to start working with cars does anybody have any recommendations on how I can start learning about cars they have always been interesting and I could see my self having a hobby working with cars but i dont know much about them. Another question I have is The handle to the glove compartment fell off so I cannot open the glove comparpment does anybody know how much this would cost to get it replaced? Should I put money into this car or save it for a new car because It has 121k miles on it? Try to answer all questions and ill give you best answer if you dont no everything then just list what you do know I really want some advice! Could you guys please give me estimations on how much it will cost to replace the speakers if they are shot and how much money I should put in towards that or if its even worth it cause its a 1999 with 121000 miles on it. Is it going to last that much longer? Do you have any recommendations for me? This will be my first car. I just basically want to start learning about my first car (1999 audi a4 1.8 quattro sedan ) and learning about cars and all the different parts and repairs I may have to make and things like that.