resistance soldering irons, how many watts is required?

I assume you are referring to a Resistance Soldering Unit Paul. They are a useful, but pricey tool for brass kit building, not many models available, the most popular being the "London Road Models" unit, which they build in batches according to demand, so you may have to wait a while after ordering unless you are lucky enough to drop on a secondhand one on Ebay or the Like. If you use the search box and put in Resistance Soldering Unit with my name Keith M as the poster, you will find some posts I made around 2020 about a cheaper unit I bought, not quite as versatile as the London Road version, but which does the job admirably. The only proviso is that you will need to make up a short lead with a foot switch and a 13 amp socket to run the unit from (unless you have 3 hands!). The unit itself is basically just a very low voltage transformer which gives a secondary voltage of 2 volts or thereabouts, but at sufficient current to rapidly heat up a carbon rod to melt solder in a joint. Solder paste is normally the best way with these units, with only a minimum of solder required, but you will quickly learn how much and how long to make the joint as you gain experience. They are a very useful tool for fixing such fiddly items as lamp brackets etc, but my usual assembly tool of choice is a small pencil gas torch. That said, due to my background as an Electrician I have many years of experience with soldering, but as with most things, you only gain that experience by doing the job, and you'll quickly get the hang of it.