HangFan - I just made a Double Layer hammock New Years Eve with the Hancock Fabrics ripstop. It is heavy weight stuff and I wouldn't have any concerns putting a big guy in the one I made (though I'm only about 165#). Mine came out at ~1.5 pounds with suspension. More about the project here
As Fronkey said, just grab whatever 100% Polyester thread they've got. Don't use cotton thread, it won't hold up.
As far as whether or not you NEED to do a double layer due to weight concerns (as opposed to wanting the DL to put pads in) the WBBB website
has a useful reference of hammock material & weight limits. Basically single layer 1.7oz should handle 250# and 2 layers of 1.7oz 400#