Should my first Diagnostic (Quant) test be timed or not?

For a first time diagnostic test taker (with about 3 weeks of studying), what is a better diagnostic test to take?
GMAT prep test from mba.com

Should it be a timed test?
Yes. You need to know not just the areas where you make mistakes but also the areas where you are slow.

My question is about the difficulty level of these tests. I surely want the test to "Test" my level of knowledge. But at the same time don't want it to break my heart this early in my preparation, especially if its unduly harder than the actual GMAT test

Once you figure out your weak areas, you need not time the practice questions. Try to be very comfortable with the concept. Rather than trying to just get to the answer, try to find the most optimal method of arriving at the answer. While you are working on concept building, timing the questions is not required (in fact I would advice against it). Once you have a certain level of comfort, you can take practice tests which will obviously be timed again.