Volume will grow to adsorb any available bandwidth/speed. Buy as fast as you can for as little as you can. You can always upgrade so start as slow as you think you can stand it.
I am using 2Mbps connection ..It is best for me to watch Youtube video fast,Downloading and uploading speed anything fast.You can check the speed of the internet using the site Scanmyspeed.com I have checked the speed of my internet using this site....You can get 1mb or 5 mb connection to access high speed internet.