Same as above response - most of your questions can be answered on the official website...
1) Best way to visit from Opera house area is to take the U4 subway from Karlsplatz (in direction Hütteldorf) to Schönbrunn station. It takes about 10 minutes and then a few minutes walking. If you will already have a Vienna card, 24-72 hr transit pass or other transit card, this trip will be covered. If not, you can buy single tickets for less than 5 euros roundtrip.
2) I would not suggest starting your visit at 5pm unless you want only the most basic visit. You will see on the website that closing time depends on the month - in some cases, it closes at 5:30 so you wouldn't even have time to go inside. Otherwise it all depends on your interest level and schedule - I am sure some people are satisfied with 60-90 minutes here, but others could easily spend half a day or more visiting the palace, walking around the gardens, Gloriette, etc.
3) If you are visiting on your own, prices and further info (including how much time required) about the 'official' tours are listed on the website. Which is one is "the best" is totally subjective and depends on your interest level, time and budget. There are also deals if you want to visit other attractions at the palace or in Vienna, so choose what works for you.