Is it a bad idea to buy a new sound system for my 1999 Audi A4 with 121000 miles on it? + Other questions?

first, start out replacing your radio and speakers. that is going to be the biggest upgrade in sound. the radio will cost you about a hundred and the install parts are gonna be about 40$. 4 good speakers are going to be around 150$. all of that you can do by yourself with a little help with youtube and google. im 18 right now and put in my first sound system all by myself when i was 16. i just watched a lot of youtube videos and googled alot of things. you have to just do your research. its a fun hobby to get into but takes patience. (and $$$) after you get your radio and speakers and want to make it better, get a sub or 2 from best buy or craigslist.you can get a 12" sub for about 100$ and a box for like 50-75$ an amp and wiring kit can be about 250$ (cheaper on craigslist). after all of that is done, add a 4 channel to your door speakers. that will unleash their true power. again, do your reserch and read alot of reviews. thats what i did.

1999 - the speakers are probably shot. Replace them first. No use spending more money than you need to, but if you replace the head unit it won't work well with crappy old speakers.