For gear, look into some boots. Any over-the-ankle boots are better than sneakers, but I prefer riding boots. Mine have a leather pad on the top of the left foot to resist wear from the shifter, and (more importantly) have ankle bone protectors to protect my ankle bones in case of an impact. My budget for gear was $1000. Used gear is fine, except I wouldn’t trust used helmets – you might not be able to detect damage that will significantly decrease the helmet’s efficacy.
As for the break-in, how a used bike (or car, for that matter) has been treated is a matter of luck. The specifics of the break-in period aren’t all that important. Actually keeping it under 4,000 rpms on some bikes is very difficult. In my non-professional opinion, it’s more a matter of not pushing the bike to its limits, not keeping it at redline, etc.
If you can drive stick and ride a bicycle, the mechanics of the bike shouldn’t be too hard. But as was mentioned above, an 18-wheeler is very very hard to miss on the road. It’s unlikely that people pull out in front of you all that often. On a motorcycle, you have to assume that every person who can pull out in front of you will do it. Being comfortable with traffic is certainly a plus, but riding a motorcycle takes a different mindset than driving a truck (or so I would imagine).
I own a Ninja (650r), and it’s definitely not an off road bike (I should have a disclaimer that I haven’t had this bike for very long). I would take it slowly down a dirt or gravel road if I had to get somewhere. But I wouldn’t take it off road for fun, only for necessity.