Just make sure we don't confuse audio compression, and data compression...
Using a lower bitrate mp3 will not make the music sound 'compressed' in the audio sense of the word, just throw away more audio information (it's a 'lossy' codec) and make it less hi-fi, introduce artifacts (or make them more audible!) and basically 'blur' the soundfield, compared to the original.
But it will NOT 'compress' the sound in the usual, audio sense of the word. For that, you still need a compressor. Depending on how dynamic and drum heavy the mix is, simply normalizing will not get your levels up to 'pro' standards. Digital zero is the ceiling, but if ONE drum spike (they are the usual culprits) gets up to zero (or close), normalizing the file will have little impact. You MIGHT be able to get away with a few db of limiting, with no comp if the track is pretty even already, but in practice, just a little mastering comp helps 'glue' the track together, and then simply wind up the limiter until the track is the desired volume.
Just don't overdo it...
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An arranger is just a tool. What matters is what you build with it..!