Depending on your budget, you may want to consider suburbs a lot closer in than Windsor. Eg: Box Hill, Annangrove, Kenthurst, Middle Dural, Glaston, Arcadia etc. I know the area pretty well (and love it!) and can tell you that I'm seeing some real bargains crop up (even in the sub $1m market) for acreage. Not all blocks are fully arable, naturally, but seeing as these suburbs represent the closest acreage there is to the Sydney CBD, land value is looking pretty good. I've cited a couple of recent examples in other posts so take a look before you settle on Windsor indefinitely. However, the upside to Windsor, Vineyard/Mulgrave and surrounds is the benefit of rail.
Downsides to investing in acreage include land tax and maintenance so your cashflow needs to be generous.
Oops forgot to mention flood zones- do be aware of these and purchase a flood zone map from Hawkesbury council or visit their chambers and look up the zoning maps- a zoning certificate will tell you more. 1 in 100yr flood zones do exist, and depending on when your house was erected, restrictions may apply on further extensions etc.