This is just a great car, as long as you go into it with your eyes open. Any luxury car (especially one going on 20 years old) is going to have issues, because of the myriad of systems on board. The HT-4100 has a well-earned reputation as a troublemaker, but with proper care it's no more or less reliable than most other composite (aluminum) engines these days. These Cads routinely go 150 to 200 thousand miles or more when maintained (check e-bay if you don't believe it).
Keep in mind also that 1987 was the last year of the 4100, so most of the serious bugs were already worked out.
Mine is barely broken in, with less than 50k, and it drives and rides like a brand new Caddy - it just glides down the road as if on a cloud. The seats are supremely comfortable, and the Delco/Bose Symphony Sound system is simply sublime. Yes, I've had to sink a few bucks into it, but so far it adds up to less than three months car payments on my Grand Cherokee. It also gets 16-18 mpg around town, 23-25 on the highway. The real beauty of Cadillacs is that they have their own on-board diagnostic computers built in. You don't even need Mr. Goodwrench to tell you what's wrong with your Caddy!
The secret to buying a good Cadillac of mid-80s vintage is finding one with a complete service history. Don't be scared off if it's had lots of things done - that just means the problems are fixed. The key item to watch for is that the engine coolant has been changed at least every two years, and the GM coolant supplement tablets have been added at each change. The supplement is really nothing more than a repackaged form of Bars Leak Gold powder, and it seeks out and fills gaps in the unstable aluminum surfaces before they become leaks. This same supplement is used on virtually every GM aluminum engine made today.
It also helps to be mechanically inclined. These cars are sophisticated, but not impossible to work on even for someone with average skills. Get a GM Factory shop manual, too. It'll pay for itself many times over.