Except for the aesthetics, the powdery mildew is usually not a problem. The brown spots are probably bacterial blight. If it were me, I'd make sure to clean the area of fallen leaves this fall and see what happened next year. If the problem persisted, I'd probably get rid of the plant. I don't like plants I have to baby too much. You might want to replace it with a lilac more suited to this area (We're not in the best area for lilacs especially now that it's warmer) and resistant to the problems.

If you do want to try to treat it, do so BEFORE the outbreak. There's plenty of advise on what to use to treat the plant if you google "lilac bacterial blight".