I am about to start with my first Stadden figure, which is also my first ever metal figure. What advice can the veterans offer to a newbie?

PS I will be painting in Humbrol enamels...

I didn't even know they were still making Staddens, haven't seen in a few years. Anyway, one suggestion which I took, and still do with metal figures is to give them a soaking in vinegar, suppose to do something about prefventing the corroding of the lead content in the figure. White or wine vinegar is fine. Do it after clean-up, and before priming.
Other than that, no other suggestions except have fun, Staddens are really nice figures.

Although I got my Stadden from an "aunt-in-law", Staddens are still available from http://www.staddendirect.co.uk - Charles' son has continued with the family business, and releasing a new figure every couple of months.

Re. the vinegar treatment - should this be used after a treatment of warm soapy water, or instead of it?

After washing, a soaking of several hours in vinegar is suppose to stop the dreaded "lead" corroision, that sometime appears after several years. This was back in the days when "Lead" figures were mostly made from "Lead" or very high in lead content white metal. The corroision would start showing up after the figure was primed and painted several years, kind of like the disolving plastic from the rubber wheels. The paint would start flaking off on your display shelf for no reason.
Over the years I believe the lead content has gotten lower than 40 years ago, where today "lead figures" are actually everything but lead. But I guess the vinegar bath doesn't hurt.
For myself, it just an old habit, so I still do it.