Another poster was asking about adding a FEL to a JD 750 on the JD forum and so I'll mention the same thing I cautioned him about. If your 850 has power steering, then you are in good shape with adding a loader. If it doesn't have power steering, then consider this....
When you add the FEL (about 400-600 lbs of iron and cylinders) and then have a full bucket load of material adding about 700-800lbs (or possibly more), then you have added probably around 1200lbs minimum of deadweight to the front end. Without power steering, you may have a difficult time turning the steering wheel!
If you plan on doing a lot of back and forth loader work involving a lot of turning without the PS, you may tire quickly of using the FEL. JD may offer a PS kit for the 850, but you won't like the price!