I'd like to welcome you to the music community here in SL, which is a very active and dynamic one. One of the best music community forums right now is:
http://slmc.myfastforum.org/index.phpAs for uploading an MP3 to SL, you can't do that directly. You can upload 9.9 second WAV files and use scripts to stitch them together, but it's sort of a waste of time and they sound pretty bad.
You can host the MP3 anywhere you have web access to it, and simply have the URL of the file as a streaming URL on an SL land parcel, but then it's possible (and likely) people will simply find out the location and download it directly.
As for in-world sales, there are a couple, SecondTunes seems to be well thought of.
As for a set-up, the beefier the better. I'm pretty happy most of the time w/ my 2 yr old Dual G5 2.5ghz Power Mac. I'd be happier, probably w/ a nice new quad core MacPro, but budget...
Anyway, I use Garageband, NiceCast and SL all at once on my machine and almost always have good results. I just play and sing myself at this point, but I used to do shows using all the source tracks from my recordings, and create a new vocal channel to sing into. Worked quite well, though did have tune changeover time to deal with. I've since heard of people loading up several songs as stereo tracks, and simply soloing the ones they need at the moment, bypassing the load-time problem.
Feel free to look me up in-world, and really look at the link above - there's a lot of good folks focusing on SL music on that forum.