If the chosen enchantment is an Aura but Estrid's Invocation won't be able to legally enchant anything, Estrid's Invocation remains in its current zone and doesn't enter the battlefield
Alpha, Beta, Unlimited, Revised, and early expansion cards can vary massively in price depending on the exact version
Chandra's Incinerator bounces Imodane's damage to opponents back to their creatures
Wavebreak Hippocamp essentially draws you a card with almost every counterspell you cast, and when thats paired with enough cost reducers, its almost impossible for your opponents to resolve a thing
All this synergy lets you go wild with other abilities that can pump up your creatures, letting you attack with a huge army for relatively little mana
Notably, you can also run creatures with Indestructible or Hexproof, since an instant lets Muddle become a copy of these creatures on your opponents turn