Can I use this to attach to the end of an existing dock?

No, this is intended to be a stand alone raft.  This 8' x 10' swim raft is supported by 4 floats and will sit lower in the water when you attach to end of a 4' x 10' floating dock section that utilizes 3 floats.   Plans are available to assemble an 8' x 10' floating section to be specifically used on the end of a dock.

Is this easy to assemble?

It assembles in only a few hours and is less than 400 lbs when completely put together.

Can the ladder be flipped up for easy winter storage?

The ladder cannot be flipped up but it can be easily removed (only 4 bolts).

What is the Net buoyancy of the Swim Raft?

The swim raft has a gross buoyancy weight of 2,000 lbs (4 flaots at 500 lbs. each).   The Aluminum Fame and Resin Dock Top weigh approximately 270 lbs. making the net buoyancy of swim raft 1,700 lbs.