Check that the wheels have 36, not 32 spokes. More is better for heavy riders.
If you twang the spokes you will probably notice variations in pitch and tension. A good wheelbuilder will even out and optimise the tension, and pre-stress the wheels so that they dont unwind. This will improve the strength of any wheel.
Make sure that you can raise the saddle high enough without exposing the danger mark on the seat post.
Some M600 bikes come with clipless pedals. If you are new and riding with kids, then swap them for some platforms for safety.
Saddles are always a gamble, they may work on some riders but not on others. Make sure the stock saddle is adjusted properly and see how it rides.