Budget of approx$500K is still not going to get you much of a freestanding home here in the Hills- I live and work here and know the area intimately. Yes, the buses aren't the greatest and a car trip plus tolls is going to be expensive but it's a great family area to live- can vouch for this personally.
Consider cheaper suburbs such as Seven Hills or Quakers Hill if you prefer rail or even Northmead/Winston Hills as you can catch a quick bus into Parra station and CBD commute from there.
As for power lines- these are NOT predominantly a problem in Baulkham or Castle Hill but you do need to be aware of them in nearby West Pennant Hills - the "cheapies" you will see advertised are usually within co-ee of one of these charming stauncheons
Crestwood vs Roxborough Park- pretty much similar areas, though Crestwood has younger housing whilst RB has the benefit of being closer in to Castle Hill and is well regarded by locals. It also has Waves pool and parkland with RB oval as well. Nice spot to walk the dog and get the kids doing their laps for the upcoming athletics carnival
Don't hold your breath for the rail link planned- until it's given the official go-ahead by the appropriate authorities I won't believe that it will happen
However, it's highly likely that, once approved, values will soar to match current comparable suburbs that have rail links to the CBD (think Epping/Beecroft prices). The Hills are a fantastic place to bring up children, and we also benefit from not only being one of the highest owner occupier areas in Sydney, but we also boast the lowest unemployment rate.
$516K is the current median for housing and, whilst low compared to other suburbs in Sydney, represents fair value for an area that has a lot going on- inc the Rouse Hill Regional Development Plan and the infrastructure changes to central Castle Hill.
Good luck with your search and happy hunting in the lovely Hills
By the way, if you don't like buses, you can always drive and park at the nearest railway line (Seven Hills, Pennant Hills, Beecroft- depending on where you live) and use the system this way. I know many Hills residents who do just this instead of fighting the M2 traffic every peak hour.