Ford dealers will have the best stock - but will also be asking top prices. There's plenty about on the private market but a full service history is an absolute must. Go for an ST2 for the best balance of cost versus equipment but check for signs of abuse and avoid any with modifications.
The ST is a popular car so there shouldn't be any problem selling it. It is available in several colours unique to the ST, and we'd expect these, especially the bright Electric Orange, to be the most popular. Buyers are likely to value non-abused and unmodified cars more.