Hi there Graceful,
You're not going to believe it but I just sold a board that I had for cheap. I only have new stuff available at the moment.
We have some MBS Core 95 (~$470)which should be plenty tough enough for what you want to do. If you ride on grass, you can use an inflatable kite, if you have foils you can ride on hard surfaces.
Grass requires a bit more power than bitumen, salt or dirt. A 6 knot breeze with my 8m foil is great fun in a carpark.
If I can help you out, let me know. The beauty of landboarding is you can do it in any wind direction in very light winds if you have the right gear.
Call me on 0433 982 696 to discuss what we can do for you or arrange a demo ride. It looks like we may even get a bit of a crew happening, it could be fun on a 12 knot day! Ride always!
Cheers mate,