I live on the other side of town from Zuffenhausen and Porsche. The Mercedes Museum is definitely worth your time while you are here, even if you aren't a Merc fan. I don't think the trains or train stations are scary but we have found that often it is cheaper and easier to rent a car. I've been to Stuttgart Hbf, Munich Hbf and a bunch of the smaller bahnhofs that are a day trip from Stuttgart. July can be warm here (not Florida warm, but warm). We live in convertible trekking pants during the summer because they are lighter weight and more comfortable than jeans. Beware, many homes, flats (and subsequently VRBOs and AirBnBs) will not have AC. It's really not that bad but you need to be sure not to overdress.
PM me if you want to have a beer while you're here
