You can actually build the house you want. There are empty lots on the market most places. That's what we did. Our house is so well insulated and passive-solar heated that here in Iowa, even with the -20+ temperatures we've just had, the furnace doesn't run all day.
The financing required to buy the land and build a new house is more difficult than buying an existing house.
Building a new house typically requires more collateral because the bank is wary of loaning the money and then the project falling apart before its finished. When that happens, all they can reposes is parts of a house that aren't even worth (on the open market) the cost of the building materials and work already put into it.
It's not a huge deal if you go with an existing builder with lots of experience, especially if they already have a relationship with the bank... but if you want to save money by being your own general contractor or do any work yourself, that's when you encounter the most problems.
