I'm afraid the real answer to your question, even from one as loyal to Mercedes as I am, is to say that the whole system of labelling Mercs is now in an amazing mess.
Things called 180's are often 2 litres. Things called 320cdis are often only 3 litres. The A 160cdi, the A180cdi and A200cdi all have a 2 litre diesel engine just different states of tune and different turbos. Best not to look for any rhyme or reason.
Back to your question about SE and Elegance. You just have to look at each and every model and check carefully the list of options and bits that are included.
A class: - offers Classic, Classic SE (which adds aircon, foglamps, air-conditioned glovebox and a better sound system), Elegance SE, Avantgarde SE and A200Turbo.
Go to the B class and there is no Elegance, or Avantgarde or even Classic. A much simpler (and IMO in some ways a better system) with just Standard or SE for most models other than the badly named TURBO (which means the 200 petrol turbo not the also available 200 diesel turbo!
)
The C class confuses with yet another variation. There is an SE model (what used to be called the Classic SE) an Elegance and a Sport.
Move on to the E class and the term SE has now disappeared. Just Classic, Elegance, Avantgarde and Sport.
When you get to the S class there is no Elegance, Avantgarde or anything. Just different engine sizes, and a long-wheelbase version, and a menu of extras. But the standard spec is pretty good.
So the truth is when Merc use the term SE it means anything they want it to mean, at that particular time, for that particular model and you need to read the price list to find out what is and what is not included.