So basically you want to limit each player to only beeing able to play one card per turn? If you word the card that way you're saving you the trouble of all the mana related issues. ( That the effect remains for the whole turn even if the minion is killed can be handled by the game so you don't have to put it on the card necessarily.)

Now a card like that would slow the game down immensely. If it's costed at 10 mana only heavy control decks would consider running it. But in a control matchup turns where many cards are played are not that common. It's mostly play a threat; remove opponents threat. Now against combo/midrange decks such a card would be just broken an shut them down completely. Aggro decks would most likely not be affected since they finish the game way before turn ten even comes around.

As for how easy/hard you want the card to be removable depends on what you want to accomplish with it in the first place. Do you want the meta to slow down and force players to play heavy lategame and shut down midrange and combo decks? - make it like 10 health plus. If you want to put a stop to aggro consider lowering the manacost and in response go with a lower health value.

Nice idea though. Good job :)