You should have your agent look at the most recent sales in the neighborhood (last 2-3 months) with similiar sized homes to get an idea of what they are actually selling for in comparison to list price.
Also, please get a home inspection. Buyers make mistakes when buying new construction and assuming that everything is perfect. Especially if the home has been unoccupied for over 2 years.
The home that you speak of having sat for 2 years is not uncommon. Builders often times construct a handful of "spec" homes to put onto the market to both keep their trades people going and to attract buyers looking for a new home that is already available.
In new construction, tax information is typically based on the vacant land and it is not fully assessed. You can visit the Auditor's website and locate the tax calculator and calculate what a fully assessed tax amount would be for that given area & school district based on your hypothetical sale price.
As for how much to offer, and how much is the value of the home... that can be determined by working with a REALTOR. Be wary of trying to determine an amount based on what might be found on Third Party sites, as they may not be accurate.
If I can answer any specific questions for you, please do not hesitate to contact me.
Art Hotes ~ RE/MAX Classic