Steve
Definitely the rotary surface cleaner is the right tool. Just make sure that your pressure washer has enough psi and litres per minute, I use a pressure washer with max 3000psi and 15lpm with a "whirlaway" 18" rotary surface cleaner. You will also need a lance for doing edges and rinsing down. Make sure you don't block their drans with the sand and debris you wash out,
Without havig seen the site, it is difficult to estimate how long it will take to clean, but I would estimate 1 man could be 3 - 5 days to clean, main factors are mains water pressure/drainage and accessibilty of the area.
The surface and joints need to be dry before you re-sand. The sand needs to be bone dry otherwise it will not fill the joints, so please do not have it delivered loose. Although silica/glacial sand is the best (less weed seeds seemingly) Any builders merchant will sell 20-25kg bags of "kiln dried" sand and wickes do a block paving jointing sand which are acceptable alternatives to silica and slightly less expensive. For 1400m2 cleaned with a rotary surface cleaner, I would expect to use 20 - 30 bags or so. You want a large headed soft natural broom head again available from any builders merchant or DIY shed.
Hope this helps.
Good luck.