Yes, the amp and headphones would be suitable for use with an MP3 Player.
The Pro-Ject amp has two RCA phono inputs and I presume your MP3 players have 3.5mm stereo jack sockets. So you would need a 2x RCA Phono to 3.5mm Jack cable. We do offer these cables. See the following link:
http://www.hifiheadphones.co.uk/cabl...-ct-59_66.html