I don't own the 840 only the 630 and 1120 , but here goes...
The S-Arc 840 is most Arcer's favourite Arc in that lineup. Good speed, pop (for jumping), wind range. Arc's in general have awesome stability, they are usually the last to fall out of the sky and must be flown into the ground or water in order for relaunch to be required. They can be backed off the water in the same manner as other ram-airs can be backed up. They have very user friendly flying characteristics (very predictable). My Arc's are used year round on water, land and snow. I solo launch and land my Arc's almost exclusively. They are an inexpensive engine to kiteboarding of any type.
For your 60kg the wind range for the 840 would be around 9- 18 knots for the water (more or less depending on skill and board selection). Might be about the same wind range for land with a buggy a little less if on a mountainboard (or same range just lighter wind). In powder snow on a snowboard I can use my kites in more wind than I can on water.