Layovers: Depending on what airline you're flying, you would presumably fly DTW-XXX-LAX-SYD. At LAX, you don't need much of a layover - the only issues you would have to deal with would be a possible terminal change, depending on who you are flying with (and the need to re-clear security) and the potential of any delay in the arrival of your in-bound flight. A 2+ hour layover should be fine.

On the return, a somewhat longer layover may be advisable. You have to clear immigration at LAX, which is an unpleasant and time-consuming process, even if you are a U.S. citizen. You also have to collect your bags and clear U.S. customs, re-deposit your bags, re-clear security and get to your connecting gate (which may be in a different terminal, depending on what airline you use). Safer to allow 3+ hours.

Provide some information on what airline you're thinking of flying and what flights, and people can provide more specific comments.

Time of year: If you want to avoid heat and humidity, then obviously spring (September - November) and autumn (March - May) would be preferable. Avoid summer (December - February). Winter (June - August) would be ok, cooler and wetter but certainly nowhere near as cold as a Detroit winter!