Just a few more considerations in addition to Arn and Matthew's thoughts below.
If you are planning on building new (either on a vacant lot or a tear down), you should plan on 24 months of construction time. I was with a client just a few days ago in Los Altos Hills looking to tear down and build new, and the local builder we were talking to walked my client through the steps (design through completion) and in the end said 24 months would be a reasonably smooth project. He also quoted $350-$450 per square foot for construction costs.
As for builders, I'd recommend seeking builders (and architects) that are familiar with all of the local building codes/regulations. Both Portola Valley and Woodside can be somewhat picky, so it really pays to go with a team that is familiar with the town where you are building. It will likely save time and headaches in the end.
Regarding proximity to 280, many of your decisions will be influenced by desired lot size, flat vs. steep parcels, tolerance of road noise and desire for views. All of these factor into the cost, so you will have some decisions to make and priorities to evaluate.
Finally, remember that not all of Woodside attends Woodside Elementary (a portion east of 280 attends Redwood City schools). If public schools are important, be sure to know which schools each home attends.
Finally, a quick plug. I have 2 kids, and Ladera has a great sense of community. Plus it is close to 280 but avoids much of the 280 road noise. There aren't any vacant lots however, and teardown quality homes don't come on the market all that often.