I have been in your shoes. I went from a tired C2 many years ago to an F diesel and from the F to an M2 and from the M2 to an N6. Yield or yield potential should be your consideration on going into an M series or N series. My M2 was a great machine with a 6-30 head, but when my yield got higher than the machine was built to take I was forced to the N6. The M2_3 with a 6-30 has a yield capacity of about 180 bpa on level ground and is reduced greatly as you get into slopes. I found that above 180 bpa that I couldn't get the M2 to run slow enough to keep the grain in the shoe of the machine. I traded for an N6(1982) and it hasn't found a capacity problem as of yet. It went thru corn above 220 bpa this Fall without any sweat. What is your current yield and what will it possibly be in 5 years will determine your choice of M or N series. Good luck!