Part two :
This is a very subjective topic but in a general sense, the golden age of steel construction (meaning that the largest volume of steel yachts were built in 1950ís through the late 1970ís) was a period, which was not one of the best for yacht design. Many of these boats were designed to boat styles with short waterline, low ballast ratios and massive wetted surface. These boats have little value as sailboats except as museum pieces or coastal cruisers. Others were built as serious traditionally styled offshore cruisers, and if that is what you are seeking then these boats are not hurt by their design style and all other factors being equal they are worth about the same as glass heavy duty cruisers of that same build quality, size and era.
Then there are the positively freaky out there. Some are good boats for some specific purpose and other are just plain poor ideas for any purpose. But either way, these unusual boats demand inherently smaller prices because even if this boat fits your needs, it will be harder to find someone who wants the same thing that you want.
Size comes into play here as well. A serious hull weight penalty is paid in boats under 45 or so feet in length. This weight penalty usually results in some combination of reduced carrying capacity, motion comfort, stability, fuel efficiency, and performance. Smaller boats therefore take a real hit at resale time as compared to larger boats because their designs are seen as being a real compromise.
What you just about wonít find are well-conceived, relatively up to date modern cruiser or high performance designs. (Yves Tanton and Dudley Dix have done designs that come closest). If you do these would make great offshore passage makers.
This is the big one. When I was working for Charlie Whittholz, designing steel boats in the early 1980ís, Charlie and I talked about the lifespan of steel boats. In a general sense they were seen as having a lifespan of 25 to 30 years without needing a major rebuild. In those days, worldwide there were quite a few yards that specialized in replating steel boats, so the idea of replating a boat was not seen as a big deal. But even in the early 1980ís these yards, were becoming increasingly rare and quite expensive. And it is not a matter of whether a steel boat will need to be replated, but when.
Much of the problem with steel construction is that there really is no good way to protect the steel permanently. While some of the newer steels and coatings greatly extend the life of the boat, by the very nature of typical steel construction there are places that simply cannot be protected. One such example is the area between the frames and the hull, especially on longitudinal framing, where water gets trapped and rust begins quite readily and over time can become a critically weakened condition.
When you talk about maintenance on an older steel boat, it therefore goes beyond the usual types of routine repairs and maintenance found on any boat that age. Just like you can expect a 20 plus year old boat to have had its engine replaced or rebuilt, or its standing rigging reaching a point of needing replacement, you should expect that a 20 year old plus steel boat will be nearing a point where the plating has been or needs to be replaced or at least partially replaced. Good long-term maintenance would have included periodic inspection and minor repairs to the coating systems, periodic detailed surveys and replacement of any bad plating or frames. This may also include some destructive testing around frames.
A 10 year old or more steel boat that has never had replating is worth considerably less than a newer steel boat (all other factors being equal perhaps a third to half the price of a glass boat of that era). An older steel boat that has had partial replating or even been replated is worth a bit more (all other factors being equal perhaps half to 2/3 the price of an equal quality glass boat)
Lastly, this is a highly subjective point, but there are portions of the world where steel boats are more widely accepted and places where they are seen as a white elephant. It seems as if they are much more popular in the North American Northwest, New Zealand, South Africa and Australia and much less popular in the rest of the US and Europe. (They used to be very popular in Europe and there were a bunch of top-notch yards building routinely in steel) As a result, the same boat on the US east coast might sell for a small fraction of what it would sell for else where in the world.