Try Audacity. I’m not an expert with it, but it should do what you want. You should also try to always buy songs that are 320k. You may be trying to mix some 128k songs with 320k songs and there’s pretty much no way to make that sound right unless your drinking.
“Mp3Gain” will normalize your files…however I would avoid any eq adjustments personally. Many of the tracks you get are carefully eq’d and mastered in the production process. I asume you would have to use a DAW of your choice and re-adjust the frequency, and export that track to mix live. However, Im pretty sure you will destroy much of your tracks dynamics by doing this. I find that careful eq adjustments and gain control from the mixer will sound better.
Im sure someone will chime in to dispute that but thats my 2 cents.