We ran a 17.5' when we had a 1640 in 50 bushel beans no problem. A late model 1640 will also give you the crossflow fan, longer seives and the Cummins engine.
The late model 1640 that I will be looking at has the options you mentioned. Do you think it would handle a 20' tableIJ I sometimes combine with a guy that has a 1660 (1991) with a 20' head. We run about the same speed in beans. Makes me think that mabe a 20' head would be too much for a 1640!
A 20' will work but you just have to drive slower. That's good in beans because you will do a better job of cutting. In wheat it might get a little slow (boring) depending on what the crop is like (45 bus. or 90 bus.) At the end of the day you will still have the same amount of acres done.