At the moment I'm still practising restoration/repair with a view of taking it up as a sideline, so this site and all your suggestions are very useful.
At present I'm using a bunch of old photos that I've collected, purely for practice material.
I anticipate that most of the potential customers will be those that have caught the Ancestry bug so historical accuracy may be less important to them than an enhanced image.
In the end, as Kraellin says, it's about keeping the client happy. Also, a pretty good understanding of historical processes is useful - something else for me to study

Well, I've started cleaning the photo now (attached) so I'll consider toning once I've finished.
Thanks