I completed my '65 set last year. I agree with what most everyone else has said: The Uecker and Hunter RC were especially difficult to find a good deal on.
Although the high numbers aren't as difficult as say 61, 66 or 67; the shear number of them (#371-598 127 cards) makes it somewhat challenging - kind of reminiscent of the '72 set.
The Mantle is a pretty popular card as well and will set you back more than any other mid-late '60s Mantle.
Check out my '65 set
here.