Either the Nacra 6.0 or the Hobie 18SX would be a good choice considering your size and number of crew. I've never sailed a Nacra 6.0, but I have a lot of time on a standard Hobie 18 and Hobie 20 (similar to N6.0). I would say at your and your son's size, bigger is better. Even though the SX18 has a slightly larger rig than a standard 18, I think you're still going to be a little bogged down, especialy if you are sailing with 3 or 4 big people. The Nacra is probably a bit floatier. Also, at only $500 more and 7 years newer, unless the thing has been run over by a cement truck, the 6.0 is probably the way to go.
As for racing, either one of these boats would almost certainly be raced in the handicaped fleet because there aren't many of either in your area. There is a small but active group of standard (non-SX) TheMightyHobie18 racers that race in the area. And there's a fairly large and active hobie fleet right in your area, up a Lake Nockamixon, fleet 416 which you might check out.