Nothing that doesn't go out on any other vehicle. I am a service manager so labor is not an expense for me.
Replaced alternator @100K. New serpentine belt while I was at it.
Brakes front and rear at 95K.
New front struts at 92K.
I converted the A/C to 134A at 97K.
I am going to replace the intake and valve cover gaskets in the next 10K miles or so. The rear valve cover is leaking a little, so I figure I will knock it all out at one time.
Note that none of the above repairs were failures. I like to replace common problem parts around 100K before they leave me on the side of the road at an inopportune time. The brakes were simply getting close to the wear marks, so I replaced them before they starting grinding so I could avoid buying rotors too. Other than that, just gas and routine maintenance.
I really like older Caddy's. The 90+ 4.x engines are very reliable and make good power. They are hard to beat for V8 fuel economy. I average 17.5 mpg around town and 27mpg cruising at 72mph on the highway! A very comfortable car that cruises great. My wife and I usually drive the Caddy on trips instead of our relatively new car because it is so comfortable. The Eldorado's have a classic look that ages very well. I get compliments on my car at the gas station pretty routinely.
The interiors do have a few more plastic parts than some luxury cars. However, this is my second Caddy of this vintage and I have not had any problems with broken pieces. From what I see in my profession, many broken interior trim pieces are the result of misuse or abuse. In other words, don't jab at the various control surfaces like you are trying to punch Mike Tyson and run. My wife was guilty of this when we first started dating and would give me a hard time for asking her to be gentle with the buttons, then wonder why my older car was so much nicer than her 10+ year newer one.
I think that most older Caddy's get bad reps for being money pits because the previous older owners are so tight with money they do not maintain the vehicle's like they should. (Ask me how I know) Other than oil changes, you typically cannot talk a older Caddy owner into maintaining the car.
The key is to get one that has been well maintained, with problems being taken care of as they happen as opposed to being ignored because the car still "drives OK".
Proper care of the cooling system with GM seal tabs is VERY important. As long as you find one that has had routine oil changes and proper coolant flushes, you will more than likely be in good shape.
Transmissions need to be serviced every 30K with fresh fluid and filter. Failure to do so will result in the tranny failures that everyone complains about. Ask those same people if they know whether or not the tranny has been serviced routinely and I am sure that you will get a "I dunno" answer. These transmissions are built very well but are subjected to a tremendous amount of low-end torque. Any abused tranny stuck behind a torque-monster V8 will fail sooner than later.
I didn't mean for this to end up sounding like a tutorial. I have been in the automotive service industry for 10+ years and have grown tired of the "Japanese cars are better" line. Any car that is not maintained, be it Japanese or American made, is going to cause headaches for the next owner. I think that most people who own American made cars don't maintain them as well because they are told that they won't last as long anyway, whereas Japanese car owners maintain their vehicles pretty well because it is an "accepted" standard. I have worked for nearly every major Japanese car maker out there, and they build a good car. However, they have problem areas just like any other car. Moral of the story, take car of your car and it will generally take care of you.