Learn English – I’ll take you up on your/that job offer if (———–)

This sentence alludes to a common device for announcing a surprising event, “I’ll give you three guesses wh— ...”:

I'll give you three guesses who the new manager is.
  Who?
Carol!
  Holy cow! no kidding?

Three guesses which children’s author insisted on writing a book about seven naked women before they’d write any more kids’ titles

The three guesses device (which is in origin a storytelling device for building suspense^†) suggests that the event is so improbable that even with three successively more far-fetched tries you will not succeed in guessing the right answer.

By contrast, “I'll give you one guess” suggests that the answer is so obvious you only need one guess to get it right. A common variant is

I’ll give you three guesses, and the first two don’t count.

^† For instance:

... when the little man stepped in shortly afterward and asked: "Now, my lady Queen, what's my name?" she asked first: "Is your name Conrad?"
   "No."
   "Is your name Harry?"
   "No."
   "Is your name perhaps, Rumpelstiltzkin?"
   "Some demon has told you that! some demon has told you that!" screamed the little man, and in his rage drove his right foot so far into the ground that it sank in up to his waist; then in a passion he seized the left foot with both hands and tore himself in two.

Usually, we use this if we are saying not to take it the wrong way before we tell the person what we want to say, and that if we are saying it afterwards:

Don't take this the wrong way, but I think you should get a proper job.
I think you should get a proper job, but don't take that the wrong way.

Your use of dashes is appropriate, but what you are enclosing in dashes is not. You are not asking the person not to take "a proper job" the wrong way. Rather, you are referring to your statement that you think he should get one. To do this correctly sounds awkward:

I think you should get a proper job, but don't take that—my thought that you should get a proper job—the wrong way.

The explanation set off in dashes doesn't add to the understanding of the request—we already know what "that" refers to—and therefore can and should be removed. (There's an example of how to use dashes for you.)