The 6421 has a CAT muffler. I don't think the non-CAT will fit, but it may with a little work. I am pretty sure the BB kit will work. It (Makita saw) is a lot more saw, with the capability to be improved even more.
Yes, although we haven't determined what/how much cutting the OP will be doing.
At $150-200, and assuming it is mechanically sound, and assuming the OP isn't going to cut but a few cords per year, I would think that saw would be a good deal. It's not my first choice, but those saws put lots of wood on the ground and heat lots of houses every winter.
I would ask...all they can do is say no, and you can always buy it, take it home, and bring it back if it doesn't check-out.
A MM helps a lot and sort of makes the weight penalty a little easier to tolerate.
Me too. I rebuilt one for a tree service a few months ago. The filter was completely stopped up and the intake side of the piston showed the lack of filter attention. It was certainly easier to work on that the 290 series. A new piston, seals, and carb kit, and that saw is back doing duty as a groundie saw.