7 knots is certainly kite-able if you are experienced. You just have to accept that if the kite hits the water you'll be swimming in.
Either a proper course raceboard with big fins and a 17/18/19m kite.
Or a Hydrofoil and an efficient 13/14m kite.
Hydrofoils take a bit more learning than the raceboard but for Joe Blogs I would highly recommend the hydrofoil as IMO much more fun and versatile

If not overly experienced best option for would probs be a bigger kite (16/17/18m) which might get you going in around 10 knots