We know this area very very well
having bought for many clients across the last decade and, along with the rest of the buying population, have seen the huge rises in prices paid particularly in the last 6-9mths. Demand has outstripped supply for this entry-level buying price bracket so expect a basic house for your $800K budget. You will need to compromise on either land size/usable yard, position (more likely to be on Sunnyholt Rd side rather than Tuscan Waters/Old Windsor Rd side) or condition. You will get one, however I wouldn't leave the search too long as you can see what the ripple effect of adjoining suburbs have done for these areas. Buying into neighbouring Baulkham Hills, for example, for even $100-200K more is now fast becoming a pipedream for buyers.
Also consider Acacia Gardens, Parklea and Stanhope Gardens if you are willing to go a little further afield. If shiny and new/ish isn't so important to you, also consider the older but more established suburb of Kings Langley. Bigger land blocks for your money but again good houses in this price bracket (up to $800K) becoming harder to find. Quakers Hill cheaper again and you will get something but for this budget you can afford to stick to the "newer" side (as the locals refer to Farnham Rd side LOL). All depends on how far you're willing to travel.
I'd suggest taking a drive through all of the above and developing a feel for the suburbs before making a decision. Good luck! (you'll need it in this market
)