IMHO, as far as the basics, any old thing that's a decent Japanese brand will do fine. Volume level, sound quality, and durability will not differ between a $40 Sony and a $300 Sony. However, possible reasons to get a "nicer" unit are appearance, size/weight, and extra features, if any of those matter to you.

I also have the Bose headphones, and use them very happily with a basic $39.95 Sony CD player from Walmart. At home, I have an "audiophile" system (those big black boxes with no remote controls), but for music on the plane, the simplest CD player is just fine, IMHO. It'll be plenty loud, especially since much of the background noise will be gone. It will be a huge step up from the in-flight audio system, which doesn't fully utilize the capabilities of the Bose headphones.