Who owns an r40?

Can't say about an R 40, but I have an R 50 and it handles 6-30's easy,  16 foot flex easy, can't think a 40 would be much different just somewhat less "grunt".

Rated engine power.   My 50 will "run out of head" before running out of capacity. 

I had a R50, traded to a R52 the first year they came were made (same as R50 but different cab).  The R40 is the same combine just less power.  You will have no trouble with the combine itself with the power it has.  I have a 8 30 head on the 52 and am real happy with this arrangement.  It gives me a lot better cleaning job as I am able to keep the combine full all the time at about 3 mph in 180 bu/acre corn.  A 20' flex head in 50 to 60 bu/acre beans no problem at 4 mph or more.  If you get one that has a low wire concave corn cob plugging can be a problem in the concave in some varieties and not in others.  The high wire concave does not have this problem.  The low wire concave is much more desirable if you do mostly small grains and beans.  I am mostly corn with 200 acres of beans and have (my Dad's combine)  an M2 for my rye and oats/wheat (about 100 acres total), and have put a high wire concave in my R52.  The R models with the Deutz engine have the best fuel economy of all the rotaries that I have run.  The Deutz engine is noisy when you are outside the combine but quite quiet in the cab.

I doubt if you find any R40 with the "high-wire" concave because they didn't start making them until after the R40's were out of production. Don't worry about that feature, it will work fine in corn/soybeans if you remove every other wire from the rear of the concave and the seperator grate. Doesn't cost any money, just a couple hrs of your time. An R40 is a really nice 4-6 row machine....not big, but will do an excellent job if in good shape and adjusted properly.