Files should be delivered to your mastering engineer in wav format 44,100kHz 16/24bit. This format ensure the engineer has a high quality version of your track to work with. Never deliver a mastering engineer an MP3 even if you are told it is high quality 320kbs, it isn't. If however the only file you have is an MP3 and there is no way of obtaining the WAV, then an MP3 can be worked with as a last resort.
Regarding headroom, ignore all the conflicting information online, it does not apply to today's modern music industry and mastering. As long as your mix is not peaking above 0db the mix has the perfect amount of headroom. Any mastering engineer can adjust the volume of the mix running into the mastering chain to their desired level. -3DB of headroom is an ideal level to work to.