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Yes, this land enters the battlefield with a +1/+1 counter, which does nothing while it's a land. When it's put into the graveyard from the battlefield, you may put its +1/+1 counters onto target artifact creature, no matter what Power Depot's card types are at the time.
If Power Depot is a creature and gets enough -1/-1 counters put on it to cause it to go to the graveyard, modular will put a number of +1/+1 counters on the target artifact creature equal to the number of +1/+1 counters on this creature before it left the battlefield.
Power Depot is a land, so it can only be played as a land. It cannot be cast as a spell.
The mana produced by the last activated ability can't be spent to activate abilities of artifact cards in other zones, such as cycling or unearth costs.
