It depends on what you would be using it for.
A) Daily driver. In that case, YES...it would be worth it to put $850 to get it back on the road. Insurance will be fairly cheap, no car payment...should be good on gas...it's WORTH IT. Just have realistic expectations. Expect to get a few more years on it, at best. But even if you keep it until say, 2020... in daily driver use, then the $850 was a good investment
B) Spare car. Fix it for $850 and then keep it mostly parked. Ummmm...
NO. Junk it. You'll still have to maintain insurance on it, just to keep plates on it. Without plates, it's useless. Also, cars need to be driven regularly, or else they start falling apart quickly...
In other words, parking it in your driveway (or even in a garage) is the worst thing you could do with it. A few years from now when you decide you might have a use for it, it will have so many new problems that you will wish you had junked it back in 2017...
Fix it and DRIVE it. Or Sell it to the junk yard now....