I was in the same position as you, a couple of years ago…..600+ acres (small grains) to get over, a full time job and a 1440 with 4000hrs. Good luck finding a 1680 / 1666 with 2500 hrs…I guess they are around but difficult to find, and all the ones I looked at still needed $$ spent on parts and tires. In the end I got a 1680 with 4000 hrs, good tires, and a bunch of new part on it. I put a bunch more new parts into it, and I finally have it running pretty good. I just ran 300 acres of wheat through it with only a couple of minor repairs on the header (30ft 1020).
I kept my 1440 as it wasn’t worth much and used it as a back up, which got me out of trouble a few times. I did most of the work on the 1680 myself and parts aren’t cheap!!....there were times when I thought I would have been better off with a $40k machine, but I still only have less than half that in the 1680 (excluding the header), and it is now running pretty good….still have a list of small stuff to do on it. I’ve seen 14 and 16 series combines running with 6 – 7000 hrs. My 1440 still runs good, but I’ll probably sell it now.
If you are looking to increasing your acres every year, you may be better of spending out for the newer machine, but beware, some of the $40k 21/23 series machines with 3000+ hrs need as much or more money put into them than the older 16 series that have alrady had some major parts changed or upgraded., and they still have their share of the little things going wrong like ac and electrics.
Good luck, Chris