Hi Steve, what part of the UK are you ?
356c price depends on condition, if they are right or left hand drive & also their history. i wouldn't part with mine for under £35k
don't worry about the engine not being matching numbers (few are these days) as long as the right age/type for the car and its in good order.
Andy Prill of Maxstead-page & Prill
http://www.maxted-pageandprill.com/services.html is the guru of these cars and engines. or talk to the guys at PR Services in Billericay essex
http://www.prs356.com/heres my babys= a 1964 356c coupe & a 1957 356a cab