All W116 the bodies and interiors are identical. of course there are 3 different seat materials. leather, cloth and velour. the SEL is 100mm longer.
the 280 was the most sold worldwide
the 450 was
280 should get better gas mileage, less total power. either that you look at just drive to see if you like the feeling of it.
Very different engines. I have a w126 300se - auto and for a short time had a W126 280se - manual. even though same body, they felt like completely different cars. the 280 is a higher reving very peppy engine. the 300se is sedate.
I would think the w116 would likewise have the same different feel to them.
biggest issues of W116 are rust, rust and rust. look under floor mats, corner of firewall and fender, around rear wheels, bottom of doors, jack points - try the jack in each hole and carefully watch to see if things just start bending....
There are many out there, look for the best interior and exterior, and not one that "ran when last parked" - those are a couple thou to just get running.
any you get need full fluids and filter changes immediately.
Euro are always better more hp and simpler heating controls, smaller bumpers. the later usa GM heater servo can be a nightmare.
because of their age be prepared to replace a lot of rubber items.
w116.org will have tons of info for you there.