For what it's worth,if they'll throw in the steering cylinder,get it. It's worth having. If not,the one from your 1600 will work. Take the cap off the top of the steering cylinder and take that big bolt out. It loosens then comes up against resistance again and works as a puller to take the gear off inside the cylinder. Take it apart that way and get the shaft for the center steering arm and all. Get the pivot block that bolts up under the frame.
That's pretty much it,that steering shaft and the pivot block. If you have those,it's a direct bolt up. I don't know that there are too many other parts that'll swap from one to the other. The 1850 had a better seat and will interchange if you want to get that. The steering hydramotor might come in handy some day. I've got a 1600 parted out. I've stripped it down pretty good and used the parts on my other 1600 and my 1550,but I don't know that I've ever found much else that would work on the 1850.