Movement (movespeed) is a fundamentally offensive and defensive stat (simultaneously), just like range. This is because it gives you the ability to "In 'n Out" a slower target, avoiding damage altogether. Movespeed should therefore be balanced such that squishy melee champions tend to get the best of it, squishy champions overall tend to get the next most, and beefy/tanky champions tend not to get more (by much) than squishy champions. There are of course myriad variations on this depending on role, kit, etc., but this is generally a good rubric.
Hecarim in particular makes a good jack-of-all-trades, which is why he is usually built as a bruiser, despite the fact that he can 1-shot most non-tanks if built properly. As far as building for MS generally, check out these items: Zeal (and derivatives), Zephyr, Ghostblade, Twin Shadows, Mercurial Scimitar, and don't forget the boots upgrades; all but Distortion gives some form of direct bonus MS, though Captain boots only affect allies (though if everyone gets them, there are some interesting implications). Of these, I find Zephyr, Ghostblade, and Mercurial Scimitar (Hg) to be the most attractive on Hecarim, though there are other items, such as Hydra, that are also intriguing.