flat5

For the project you mention, a 25 watt is best for the printed circuit board work. If you have to solder a large heat sink then a 40 watt might be best for that. However, a headphone amp will not require that, I think. You can buy a cheap iron at Radio Shack or wherever. For one project you don't need a $150 variable work station!
I find a glass ashtray useful as an iron stand/holder.

As stated above technique is very important.
Keep the iron tip clean. Use a damp sponge or cloth. Do not use a file or scrape the tip because it is coated with copper. You file it and the copper is gone.

The procedure is, wipe the tip and then apply a little 60-40 or 70-30 rosin core solder to it right away before the copper oxidizes.

You can Google how to solder correctly.

Let us know how the project goes.