Ing. stgands for Ingenieur, and I see no indication that this necessarily implies a doctorate in engineering in my references:
Duden, Deutsches Universalwörterbuch, ISBN 3 4511 05503 0
Bertelsmann, Lexikon der Abkürzungen, ISBN 3 570 01604 8
Duden W. der Abkürzungen, ISBN3 411 02741 X

In the US at least, titles are usually omitted in the form of address -- except for Dr., as in Dear Dr. Schmid. (When in doubt, the Dr. is sometimes used anyway, for instance when a prodiminantly medical audience is addressed automatically that might include some researchers or physicists with "only" a master's degree.) Is there any context to suggest that this individual might have a doctorate in engineering rather than a Diplom? Then of course Dear Dr. Schmidt would be appropriate.

HTH Tom