what's the difference between HP 60 and HP 80

The HP 60 Puts out 60 liters of air per minute, and the HP 80 puts out 80 liters per minute. 

does the HP 80 come with AC cord?

It comes with a USA standered three prong plug. 

Is this a new pump or rebuilt pump for 265.00

We only sell new products, nothing is used or rebuilt.

Does this have an alarm?

This one in particular does not have an alarm, but we do sell the Hiblow HP-80A which does have an alarm, if you are interested in one with an alarm.

what would i need to aerate a 2 acre pond that averages 4 ft deep? how much electric does this use?

Very little about 71watts so 6 to 7 dollars a month in electric cost.

Can the air output be directed to a 4 port manifold, and then sent to separate diffusers to aerate a 1 acre, 6 ft. deep max pond? If possible, is there any issue with not enough pressure, air, backpressure, etc. that I should be concerned with?

I would bump it up to a HIblow 120LL to do a four-port manifold at a minimum and be more comfortable with an hp-150

How do I tel the model of my air pump? It looks like Hiblow HP-80-0110 shown on the picture, with an additional enclosed vertical panel and red light on top. Mine stopped working and I'm curious whether it can be serviced or if I need to buy a new one. I'm in Altadena, CA. Thanks!

You need a HP-80A or a HP80 rebuild kit.