WAAS is not the same as DGPS; WAAS is enhanced stand-alone GPS (basically it uses a lot of research data to minimise GPS errors), DGPS uses comparison to a (relatively) nearby GPS receiver in a known position. Both enhance position, WAAS less so, but doesn't require a second fixed groundstation.
The AERA 500 does WAAS, but not DGPS.
WAAS is a US solution to enhanced GPS, EGNOS is a European solution to enhanced GPS. They work in similar but not quite identical manners.
So far as I know the AERA 500 does not have EGNOS capability.
WAAS+DGPS would give fantastic accuracy. But, you need some fairly specialised kit to do that.
G