It is my opinion that a Commander is prolly gonna bite it if he gets into any kind of serious CC. I used to like a PF on my Commanders, just to fight off those 3 pesky Tactical Marines that always seemed to kill him, otherwise.
So I'm not a fan of Celeritas.
Big 10/4 on Kabe's Herald. The only real value is to allow a mass first-turn Fire and Smoke. I mean, if you were running squadrons of Russes in addition to the Commander's squad... and wanted to use mis-matched models... and uh... Well, the Fire and Smoke is handy if you go first.
Iron Left could be useful against Orks and Necrons. Except the best armour in both armies is a 2+, so meh. CC upgrades on Guardsmen don't work these days.
Standard is best for a sit'n'shoot group. Something protecting some artillery, for example.
Volkov's cane is indeed, the best relic that the Guard have available. Mostly because it's so difficult to reliably issue orders to Basilisks.
Wraith of Cadia is the best-value gun upgrade in the game. Hands down. It makes me happy just thinking about it.