Hello & Welcome, Writlov14!
Yes, it is advisable to avoid making significant decisions in the midst of a depressive episode due to the skewing cognitive effects of the illness. Of course, sometimes it's unavoidable. Regarding your engagement, this is not a decision you are making on your own; you are making the decision in close consultation with a loved one, who is not depressed, yes?
Certainly. If you are planning to marry eventually, your partner would do well to have a knowledge of depression in general and of your experience of depression in particular. Something to discuss is the timing of significant events - engagement, parties, wedding, honeymoon. You may need to take into account the episodic nature of your depression and plan things for times when it is less likely the illness will interfere with your joy.
Absolutely!