Nissa's middle ability can target a land that's already untapped or that's already a creature.

Effects that modify the power or toughness of the land creature without setting it will apply to its new base power and toughness no matter when they started to take effect. The same is true for counters that change its power and toughness.

If a card in a player's hand or graveyard has in its mana cost, X is considered to be 0.

Nissa's middle ability overwrites any previous effects that set the land creature's power and/or toughness to specific values. Other effects that set these characteristics to specific values that start to apply after the ability resolves will overwrite that part of the effect.

The creature card's mana value is checked only in your hand or graveyard. You may put a creature card onto the battlefield whose mana value becomes greater than the number of lands you control once it's on the battlefield, most likely because it enters as a copy of another permanent.

Nissa's middle ability doesn't remove any abilities the land creature has.

Whenever a land you control enters, each landfall ability of the permanents you control will trigger. You can put them   on the stack in any order. The last ability you put on the stack will be the first one to resolve (As a result, you can have those abilities resolve in the order of your choosing.).

A landfall ability doesn't trigger if a permanent already on the battlefield becomes a land.

A landfall ability triggers whenever a land you control enters for any reason. It triggers whenever you play a land, as well as whenever a spell or ability puts a land onto the battlefield under your control.