Hard to say, street cocaine varies so much and imho is mostly crap. At least, that's what all the "old heads" tell me.
In the interest of harm reduction I will try to answer your question though.
My advice, take whatever you have, and measure out a very tiny amount, a "bump" as people call it. Do that little bit however you intend on taking it, wait for the effects (you'll know) and from there you can gauge what is safe for you. I must stress for
you, what's okay for you might be too much for someone else.
Now, that said. Don't expect to get anything near pure, unless you have like an amazing source or something. I'm no coke expert, but I hung around some people that did it regularly at one time in my life. I got offered a lot for free and most of it didn't even seem to be "real", that is, I'm pretty sure I was tooting something that had very little real cocaine in it. But again, I'm not expert. Its just...I've done batches that were a rush with little after effects, and I've done batches that had me up all night like I'd taken adderall. You never know what you're getting, and its expensive just to find out if your source is any good.
For your second question: I've taken a benzo (xanax) to help come off that "speedy" coke I did those few times. I did not have any problems, but, I didn't take enough of the stuff to get shit faced. I found 1mg of xanax to help with the come down, but, this wasn't needed when my friends had the "good stuff". I'm not going to comment on the overdosing, if you are smart this won't be a problem in the first place. All I'll say is an overdose on cocaine would come fast and hard, and I doubt any type of pill would help, in fact, by the time the pill kicked in the coke would have probably worn off or already killed you.
I hope you found this helpful and please, be safe. As they say, Cocaine is a hell of a drug.
