We may need to sort out some meanings first. Terminology is important, especially when quality and money are being discussed.
You have made it a point, it appears to me, to stress that quality is very important. In the general millieu of the hobby, the term has different meanings.
Quality in mechanisms to make the trains go? Probably the best are German. Quality in terms of looks and fineness of details, with lots of realistic details? I would recommend you consider the expensive brass market. Those trains are made in Korea...mostly. Think $2500 for a quality brass engine in some cases, less for some manufacturers, and the current used brass market on eBay is a buyer's market like never before.
You are highly unlikely, if I read you correctly, to appreciate any claims of 'quality' in what you term a "train set". What we call a train set are the boxed compilations of a locomotive, two or three cars, a power supply, and some track...they sell lots of them at Christmas. I am happy that you understand already the likely need to purchase items separately, because the quality is often inferior in these train sets.
The most popular engines are made in China. There is a general problem with quality control across the HO hobby in the market where items run from $400 and on down. None of what we call the better manufacturers/distributors have sterling track records except that they all do a determined bit of work to honour fairly generous warranties..and those departments are busy!!! My point is that, when you get a good engine, it will almost always serve you a long time. The heavy majority of us purchase one of these items and have no problems. But enough of us do have the odd dud that, frankly, it is a bit of an issue in the hobby. And, since it is all made in China, you will have to go European, brass, or build your own...or get a custom build. Expect to pay many hundreds of dollars if Chinese HO train items is a show stopper for you.
-Crandell
