Since you said small modification I think you have a target price on what you want to spend. I think if you want to buy quality products it will cost you 1k and up for parts. If you want to cobble something together you might be able to get away with less. I would do a search on Technic posts as he has some suggestions for low cost systems.
I think he has one in this thread
http://www.e90post.com/forums/showth...&highlight=BSW
Because the speakers are 2 ohm you can't just drop in a 4 ohm speaker ,which most are, and not cut down the volume by half. And also if you amp the stock speakers you run the risk of blowing them if you put in a significant amp.
http://www.e90post.com/forums/showthread.php?t=368847