Owning and servicing a 500E are not much (if any) worse than any of its other E-class brethren. (I've owned a 300E, 400E, and now a 500E) The 5.0 motor lasts forever as it isn't strained much with the W124 chassis. The car goes like raped ape and is the most collectable MB of the 80's and early 90's. Finding one may be difficult but worth the hunt. (if you need some help I know someone who is GREAT at finding top notch pre-owned)
Unless you break suspension parts or incur body damage the pieces are not exotic by any stretch. A recent article in European car featured the 500E in great detail. It probably isn't at newsstands anymore but you might find one anywhere they lie around for more than a month or two.
500E's are a little worse on gas mileage than other W124's. Mine gets about 16-19 in town and 21-22mpg on highway trips. My 300E and 400E both got about 19-20 in town and 24-26mpg on the highway. If your concerned about gas mileage more than performance then the 500E isn't for you anyways. j/k The body parts (fenders, front air dam, etc) are one of the few exclusive things to only the 500E. They will be expensive to replace, but you will likely have auto insurace. The Porsche designed suspension is unique, but not outlandishly priced and unless you are a menace to curbs it shouldn't be a problem. A 500E is mostly a W124 E. Its "E" and "Z" (15K and 30K) services are almost exactly the same as a 400E's. Some of the interior bits are different, but nothing that is really a common wear item. Most of the engine itself is sourced from the W129 (SL) or W140 (S class). Ditto for most of the driveline.
One thing you wont see in a book about 500E's. Their owners paid big bucks and were a unique crowd to begin with. Hence most of them took VERY good care of their cars. Very few ever hit the used market until the E55, C43, etc came out recently. The unpublished downside to used 500E's is that the gonzo power on tap got many a owner in trouble with accident damage. Have ANY potential purchase looked at for front or rear frame damage. (have seen em with both for that matter) That is why I enlisted a professional whom I trust to find me a car. It was basically custom ordering a used car.
If you have any other questions please don't hesitate to ask. The 500E will be faster than about any other MB in the lineup (except maybe the E55) so power will NEVER be a problem. (unless you think there can be too much of it) The ride is quite comfy but isn't the Caddy wallow! You should be able to pick up a very nice 500E for 40 grand.
A S500 (W140) is another good option. If your going to go V8 in the W140, your better off going with the S500. You can try a diesel, but if he thought a 300E didn't have enough power then you probably will want to look elsewhere. The 400E/E420 is a very good option that will not cost as much but be alot of fun. All of them need wider tires or tires+wheels, but a very good car none-the-less. A W210 E420 is also a good choice. The W210 has more of the big car "feel" to it vs the W124 and certainly has more interior room. It might be the best compromise for someone coming from Caddy country. Those W140 S cars are BIG, they didn't earn the nickname "Panzer" for nothing. I love the W124 in just about any iteration, but I could understand how someone used to the um...land-yacht nature of Caddy might not feel at home in one. The 5-speed automatic in the W210 (97 only) E420 is light years ahead of the 722.3 4-speed from the W124 V8's. Test drive as many cars as you can. Your own (or his!) butt will tell you more than hours of research.
PS You can always check out the Mercedesshop "Featured Cars" reviews for more info. If you would like the name/contact info for the individual who found my 500E I'd be happy to pass it along.