The JD 606 is the medium Duty rotary cutter. The heavy duty of that age would be the JD 609 and the light duty would be the JD 603.
I have always felt you get what you pay for in rotary cutters. There are many GOOD brands that make different duty cutters. JD, Rhino, Land Pride, Bush Hog, to just name a few. A good quality cutter will last you a life time with normal care.
A good JD 606 will bring $1000-1200 around here.