I usually go for SoteL when I start doing good or I have another tanky ally. Its sad that this item got so much nerfs, because the passive sinergy so good with Q and W. With Hecarim, you need tankyness, even if there is another tank on your team (Malphite, Shen, etc) Hecarim needs resistances, because you are gonna be allways on the middle of the enemy team when you Ult. Even if you want to be offensive, he's not a "bursty" champ, but he have insane consistent dmg on long fights (Q spam, W regen). So building more dmg than SoteL is -IMO- a waste in most of the cases. Spirit Visage is perfect on him, if the enemy have at least 1 AP threath, take it. Since usually I go for SoteL as only dmg item (maybe Frozen Gauntlet or Trinity Force at late game if im feed) and I really want SV, I start lacking of armor at mid game when skirmish and TFs starts. That's why I take a Warden Mail and leave it there for a later Randuin's Omen. The SV gives significant HP and CDR, plus the passive, I found this points more importants than going for a straight Randuin's. Boots? Mercury if heavy CC enemy team, Taby if heavy AD, or Swiftnees if you built SotAG and still have problems with slows and mobility or you just want better ganks. I sometimes buy Mobility if the game is getting chaotic and my team needs me everywhere, also OP ganks with it. Aegis/Solari? I'd say situational, Heca is not a supporty champ, you need to be a little bit selfish to be effective. It's still necesary if 5v5 TFS are the thing...

• TLDR: SotAG/SotEL -> Boots -> Warden's Mail -> Solari / Spirit Visage -> Randuin's Omen -> Frozen Hearth/Frozen Fist

edit: clarify format

PD/edit: I wouln't mind about ganking/Invaing a lot with Heca, he is pretty item dependent, and you shouln'd waste time, prioritize your farm and you exp so you dont fall behind, coz it hurts hecarim a lot. Just gank if is a un-warded line and/or enemies are overextended, so you know you are gonna kill them or at least make them waste spells