This is the M17 Austro Hungarian Helmet, you can tell this by the Chinstrap fixing points, witch are higher up on the skirt, whereas on the German version, it will be down at the bottom. You also can tell by the brown paint. It is an M17 because it has the Steel Liner band aswell. So it is Austrian, not German, and it is not M18, but M17, as you guessed. Personally I would say that it is Late 1917, possibly even an early 1918, because of the Leather chinstrap, of course its entirely possible that the strap has been replaced by someone over the years, but it looks original. Overall I would say its a good piece, even with the Chinstrap. The liner and pain are nice, something that is hard to find in an A-H helmet. Overall, on the current US Market, I would put a value of maybe $800-$900. Very good piece.