*** As one who teaches soldering regularly and also builds very many high quality locomotives I have to disagree, sorry.
The important thing is to get heat to the joint at the right level for the joint to happen as soon as possible so the whole mass of metal is not heated. A powerful Iron will always be better in that respect than a low wattage Iron.
the ONLY time you will create problems for yourself is by having a low wattage Iron - a low power Iron will be slow, therefore greatly increasing the risk of a meltdown.
Its cleanliness, correct solder and flux, the control of temperature and very much the ability of the Iron to deliver consistent energy to keep the tip at the required temperature thats important.
For example I use both a 55 and a 70 watt temperature controlled Iron for almost all jobs - and have even used a Gas Iron for soldering to very large whitemetal castings.
With whitemetal, 25 watt is marginal for all but small castings.
Like Peter I also do the occasional whitemetal job at high heat ... but generally have the tip at between 195 and 210 degrees for WM depending on the whitemetal itself - some has different melting point to others, but its all between about 225 and 250 degrees.
You should expect to pay about the price of a high quality locomotive for a good temperature controlled Iron - the discount models are OK for general use but will soon show their limitations when high quality work is needed.
The brands I recommend most highly are Hakko or Antex. Both ccover most of the important issues well however Hakko are without doubt now the better Iron and they are also the most sophisticated.
Their new 70 watt FX series have the element and the temperature controller as part of the tip itself, so they are incredibly stable.... plus an automated sleep control built int the stand to increase tip life / prevent tip damage from long term non-use under full heat. (and their 70 watt Iron is actually far smaller than you expect - smaller than a 20 watt Iron in fact... yet it weighs ony 44 grammes - a delight to use as its so nice in the hand.)
They are however quite expensive for the average modeller.
The most economical temp control Iron with the requisite quality is the Antex TC660 - a nice Iron at a reasonable price.