Is this a succulent plant?

Not strictly a succulent, but it should dry out between waterings.

How should I take care of it?

You will find plenty of information on the web.

Does the plant do well at a window facing south (should I expose it to
direct (summer-)sunlight)?

Yes, in the winter. In the summer it should be outdoors with a little shade.

How often should I water it - especially during the summer?

Whenever the soil is partly dry.

Should I remove the metal string, pressing the stems or roots together just
below the foliage?

Yes.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)