Love my toys just like Mr P!
Best bet is Flexifoil for a buggy. Easiest to get hold of and to get spares for to be honest. Get one with a back support board. My one is first geeneration and so just had the seat. 30 mins of using that and your stomach muscles know about it!
I steer clear of Flexifoil 4 line power kites as I always found that they are more prone to loosing air in their tips and causing them to turn in on themselves.
I use a Predator 3.8m. Its not as big as some kites but is extremely stable and predicatable. Although I am not exactly a small chap (weight wise!) its fine for me in anything over a steady Force 2/3 and got me up to 35mph at Blackrock sands a while back (which when you are all of 3″ above the ground feels fast enough!).
You are more than welcome to borrow it if you like though. Off at the PDS from tomorrow but I can put it out and you can pick it up from mine as Rach will be home looking after the brats whilst I am away!
Drop me a line if you like.
Pete