Well usually its the record label who will ask other artists to remix a track for you. If its unsigned work don’t even bother to send it to someone who is signed unless you know them personally.
As for what you send them, a remix pack usually consists of all the MIDI parts of all the synth lines as well as the processed .WAV files of the same synths and any FX that are essential to the character of the track. Drums are usually excluded.