Published load capacity for H17 is 400lb. As MBounds pointed out, passed 350lb the boat gets sluggish and you will have to really watch your weight distribution. If you and your crew is less than 250 lb, you may be better of with H17 Sport. Otherwise, go with H18 and keep H16 for going single.
Can I drive this boat up on the beach or does it require special handling?
Both H17 and H18 are pretty rugged for handling. H17 has advantage of centerboard, which retracts if it hits sand or underwater obstacle. H18 has daggerboards which are obviously not as forgiving.
H17 will give you a flexibility of going single or with a light crew. With H18 you will be limited to going with a crew. If you and your crew are on a light side, say less than 250 lb, you may be limited to light to medium air and righting H18 may be a challenge.
is it physically bad for the boat?
That would be cross-bars. The are build lightly and are known to develop cracks, if H17 gets pushed too hard.