Power Depot is a land, so it can only be played as a land. It cannot be cast as a spell.

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.

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.