Only one Fiend model would get the Unholy Might upgrade, but I see your point. When the Fiends hit, they need to kill as much as possible. Giving one model twice the chance to pen has to be win.
I love the Chariot of Slaanesh for its cheap cost and amazing flexibility. It can handle vehicles and infantry. I imagine Bill might have a kind word or two for Khorne heralds.
Daemon Princes are tricky, because their upgrades are so damn expensive. Most of what's shooting them is invulnerable so MoT for 4+ invuln is good. Considering Psybolt bolters wounding on 4+, that's not something I look forward to. I like the bolt princes, but Nurgle princes would last marginally longer.
With some tweaks...
HQ
Herald of Tzeentch, Chariot, Bolt, Master of Sorcery - 100
Herald of Tzeentch, Chariot, BoT, Master of Sorcery - 100
Herald of Slaanesh, Chariot, Unholy Might - 75
Herald of Slaanesh, Chariot, Unholy Might - 75
Elite
Fiends of Slaanesh x6, UM - 190
Fiends of Slaanesh x6, UM - 190
Fiends of Slaanesh x6, UM - 190
Troops
Plaguebearers x6, Icon - 115
Plaguebearers x6, Icon - 115
Plaguebearers x6 - 90
Pink Horrors x4 & Changeling, BoT - 100
Pink Horrors x5, BoT - 95
Heavy
Daemon Prince, MoT, BoT - 140
Daemon Prince, MoT, BoT - 140
Daemon Prince, MoT, BoT - 140
1830
This list doesn't appear to be dissimilar from other popular Daemon lists.
Another idea is to swap a Bolt Prince for two Screamer x5 squads (80 points). That's bring the list to an even 1850. I'm not crazy about Screamers myself, as I can never get them to work properly.