I usually go Spirit Visage, Iceborn Gauntlet/Frozen Heart as my core when I play Hecarim, giving me 40% CDR as early as possible and build around the enemy for later game items. (Frozen Heart/Iceborn Gauntlet, Last Whisper, Atma's Impaler, Frozen Mallet, so on.) Boots really depend on personal preference and enemy team make up.
I go 9/17/4 in masteries.
Runes: 3 Quints of Move speed, 8 Scaling armor seals, 1 CDR seal, 9 CDR glyphs, and 9 scaling phys dmg marks. Giving 4.5% move speed(6.5% with masteries), 6.18% CDR (10.18% with masteries), 22 phys dmg at level 18(34 wiith masteries), and 22 armor at lvl 18.
And that's the way I roll with Hecarim. Hope it helps.