Hi Peter and welcome to the forum. Great to have you on board. There are a growing number of us this side of the pond and Steve has pointed you towards the UK specific area of the forum. The UK group is fully integrated into the whole forum and is seen by all members and many of our US colleagues are active contributors. We do, however try to tackle some of the challenges that are more specific to the UK and Ireland.
I am from Kent and so probably not too far from you.
Looking at the photo that "rust" is nothing top worry about. All you will need to do is wipe the inside down with a cloth soaked in cooking oil and that will all go. It just looks as if it has been standing for a while. As the WSMs are porcelain-enamel coated you shouldn't need to touch it up with paint unless there has been physical damage - which there does not appear to be.
What are they asking for it? If it is inexpensive then snap it up as they go fast. I also see that there is a ProQ 22" for sale for £150. This looks in good condition too and would be worth considering