Learn English – “in a mood” or “in the mood”

It is informal, and I understand it to express impatience, i.e. to mean something like “right now”. The New Oxford American Dictionary has:

(informal) used as an intensive after a word or phrase to express impatience: enough already with these crazy kids and their wacky dances!

I don't think there is any implication of impossibility per se, although the fact that the statement is hypothetical might bias its use toward the impossible, e.g. "if I were you I'd buy the cheaper brand."

In constrast consider "if I were to get one I'd buy the cheaper brand." There is no indication that I won't "get one", in fact I might be seriously considering it.

[EDIT] Anicul adds:

The first sentence is an example of a present contractual conditional statement and the second is a future less vivid conditional statement. Present and Past contractual conditionals are contrary to fact, while future less vivid conditional statements imply uncertain potential for action.