I've owned my 560SEC for 9-years and gone through just about everything on it including tackling new leather at home. The first years of ownership were VERY much "love-hate" as I bought a very unloved car and spent years addressing the neglect. Refurbishing the R12 AC was close to $3k alone with all the vacuum systems.
I find buying new genuine OEM parts to be fairly reasonable and fortunately the car can be maintained at home with mostly basic hand tools. Now that everything is sorted it is actually a reliable car to drive and can sit for over a month and fire up without any fuss.
In terms of driving experience, the M117 (5.5L) in the SEC is lazy as is the autobox, it is not a thrilling or exciting car to drive however it is well composed, comfortable and like any large German car feels very stable at highway speeds. The thing to remember with the 126, is that the R&D dates back to the '70s and it shares many suspension components with the W116, as such it drives and handles like a car from that era.
My car has 156k on it, but it looks like it only has 50k, the build quality is incredible and it has aged perfectly. Styling wise, it is one of the best looking Coupes and GT cars ever put into production. My car is lowered with larger wheels and the Gen. 2 AMG body kit, so that really enhances the body lines.
The 500E will definitely feel like a modern more enjoyable car compared to the SEC. Brakes, interior, suspension, throttle response...you name it, will make the SEC feel a bit wooden. I honestly think the M119 in the 500E is the best V8 MB ever built with 4 cams and 32 Valves, it is a wonderful engine that really is not working particularly hard and has the ability to log 200,000+ miles while being reliable. I own a '98 SL500 with this engine and it is silky and offers good good power. As stated above the 124 chassis is fantastic and also timeless and the lineage and R&D to Porsche will always make them truly special cars. High mileage, neglected one are easy $15k+
Interestingly SEC prices are starting to really climb, so which ever you decide, you will be shelling out some cash for a nice one.
If you decide on a SEC, I have two friends that own several SECs and are now thinking about thinning their collection down and can put you in touch with either of them.
For example this one has EVERY bit of maintenance addressed and is really nice for $6500, try buying a 500E for that:
