a terrible idea for why?
Many reasons. All learned the hard way by various foreigners.
we can buy in her name start the build and then consider putting my name on down the line.
That route means you have to pay double taxes and fees. Actually, more like quadruple, as the future ones will be based on the value of the house and land rather than just the land.
Being married to an EU citizen gives me all the rights of an EU citizen does it not?
No, it certainly does not. For example, EU citizens can work in Poland without getting any paperwork. The spouse of an EU citizen needs to get temporary residency and that can take up to two and a half years with the Wroclaw office. And you cannot work until you get your temporary residency.