If I want to calculate areas with $area in QGIS in metric units (sq. unit) of a vector layer containing polygons represented in WGS84 lat/lon, I must reproject the layer onto an equal-area projection. According to some answers, it doesn't matter onto which projection I use. The results will be the same as long as the projection is equal-area, same units (e.g. meter), same datum and ellipsoid (e.g. WGS84) with small errors due to floating points and round off.
So I can reproject polygons in European countries with Lambert azimuthal equal area, or transverse cylindrical equal area, or Albers equal area, is that true?
I read many related questions especially whuber's comment