The long shoe was offered in 1990 models, but could be put on older ones. The Cross-Flow fan can also be adapted to the 1680. The 1680 is a good machine that will definitely outcut a 1480. The cab is certainly better than the 1480. We had the long shoe and good fan and it would run circles around the 1480. We were using a 24' foot 810 header (a holdover from the 1480 that the 1680 replaced), though, so your results may vary.
That being said, our 1680 was an '88 model (Granddad bought it used) and it was sort of a problem child, starting with a connecting rod that decided it didn't want to be in the engine anymore. I think it had the old DT466, though. Our dealer put in a 8.3 Cummins and we never had any problems with it (the 8.3 was used in '89 and up models). We seemed to have a lot of problems with the bin unloader drive. In corn there was a tendency to break shear bolts if the unloader was engaged too fast. I know there were a couple other issues, but I can't think of them off the top of my head (it's been a year since I've ran it).
I know in '89 they beefed up a lot of the components, which should solve the little things that always plagued us. I would have no problem recommending an '89 and up 1680. If it doesn't already have it, put in a Cross-Flow fan. It's worth the money. I think a 1688 might be more reliable, but in my area they're hard to come by. A 1680 that's well taken care of should be a good machine.
Well, my brothers tell me I'm wrong. They say our 1680 was a '89 or '90 model, which makes sense because it had the full bin indicator on the steering column. Oops
I guess I need to mention that it was '91 when they updated the unloader drive to a fully enclosed type. The '89 will still have the open gears. If you're gentle with engaging the unloader, you shouldn't have too many problems with it. I still think my general point stands, though. The 1680s are good machines, ours must have been a fluke. With only 1850 hours the one you're looking at should still have a lot of life left in it.
I had a 1987 1680 that was an awsome machine, I can't say enough good about it. I bought it used in 98 and traded it in 05. It handled a 1020 25' with ease. If I was going to keep it any longer than I did I was considering getting a 30' head for it. It had the old style fan and short sieve but I always had a clean sample in wheat, soybeans and corn.
We have been running a 1992 1680 for 3 years and have found it to be very good. There have been little repairs, but nothing major. One item that seems to need to be done if it has not already been done is replacing the drive coupler coupling the engine to the hydrostat with a greasable hub which is a decent amount of work. The other thing we have found is that unless you get an extension to raise your air cleaner up about 3 feet you will find that it plugs very quickly, and must be changed. We have been running a 30ft draper header and for the age of the combine we find that it covers the ground pretty quickly. From everything I have seen I would try to avoid the units that came out before the cdc engine was standard I believe anything before 1989.