Archangel's doesn't give Singed that much tankiness (only 250 health when totally stacked). It's giving you a total of 140 AP, assuming you get about 3000 mana. That's a 50% damage increase on his Q (per second of course) for not having much tankiness to it.
Might be worth it, but I wouldn't call it OP yet.
Nunu's W will be worth more gold than it was before (it was about 1500 gold worth before, so now it'll be closer to 2000). But I doubt he'll be super OP. Strong, as always, but not super OP.
Now go back to buying your Elo, gladly.