I'm 5'10" 225 and I solo my 18 much of the time. Coming from a H14 Turbo, you wouldn't have any problem whatsoever solo'ing the 18. You will of course need the assistance of beach wheels or a trailer to help get the boat down to the water. Personally, I wouldn't want to have to raise the mast on my own every time I went sailing (not an issue for me as I keep it stored mast up on the beach all summer) but there are plenty of people here who do just that.
As for sailing with crew, my buddy goes around 6'2" 210 and the boat doesn't have any stability or speed issues despite our extra "ballast". I've also brought along my wife and two kids and we always have a blast. The H18 is an excellent cat!
'82 Hobie 18 SE with '85 Nationals Prism (White) sails
'73 Laser HID# 3463