It's a common mistake to think that the Avantgarde was more expensive. It was positioned as a "more modern " level of trim, whereas the Elegance was positioned as something more...."elegant." The clue is in the name
Previously I had an E55 (W210) with Avantgarde trim, sports suspension, and 20" wheels. It was so uncomfortable in South West London, I vowed never to buy AMG or sports suspension again. (To be fair to the E55 the mistake was running AMG suspension AND 20 inch wheels in London)
You get a softer suspension on the Elegance and harder suspension on the Avantgarde. Broadly, wheel sizes tend to be slightly larger for each model on the Avantgarde, but they do vary from model to model
I believe dashboard and seat leather is limited to black on the Avantgarde, but may be black, grey, or beige leather on the Elegance.
Door handles of the Elegance are trimmed with chrome. Dashboard and door panels may be grey or beige on the Elegance, but will normally be black on the Avantgarde. Elegance uses wood trim, avantgarde Aluminium. There's a higher arm rest on the Elegance.
The S211 E class should be much quieter and smoother than the Rover 75, with maybe another 50% worth of load space. (Note that the back seat bottom cushions flip forward to give you a proper flat surface when the back rest goes down - just in case there's a reason why you're called Mr Mirrorman)
I think it's always 4 power windows on the Elegance, but back power windows were an additional option on the Avantgarde.
Don't - under any circumstances - by a manual E class. It's rubbish but fortunately rare.
The only way to judge whether an engine is suitable is to drive it yourself. With all due respect.... to the Rover 75... if you've been happy with the engine on that, you will be happy with the 220. Bear in mind that most of the Carriage trade running in and out of the airports use the 220, and they do know how to get a lick of speed out of a car.
Talking of which, the only real caveat about buying a 220 is that they can do a quarter of a million miles without showing it. (European airport taxis are the most obvious place to see it). So satisfy yourself that the history is genuine. A privately owned car will be immaculate and perform without fault. An ex-carriage trade car with 80k on the clock and 180k on the engine might present you with a few bills. (Although they're still insanely reliable cars compared to a Rover)