Chain saw mills take a LOT of power to do anything useful, you are probably looking at power heads in the $1500+ price bracket.
Yes, that is PLURAL power heads. For any worthwhile production it is a double ended bar with a head at each end.
Bars aren't cheap either, it is HARD dirty work at ground level (how's your back ?) with 2 stroke fumes.
I have a portable mill, it is a swing blade type.
Oh, sure the kerf is a bit wider, but the cut is straighter so there is less loss when planing to compensate for wavy blades and a 26 inch diameter carbide tipped blade lasts a lot longer than a band & is quicker/easier to sharpen anyway.
Re knots and nails;
Know the height at which most humans can pound a nail into a tree for hanging bird houses, yard sale signs, whatever, also screw eyes for things like clothes lines. That will take care of 95+% of the metal you are likely to find. Check for rust streaks too.
Slow down for crotch and knot areas, you should see them coming (-:
This is where band mills REALLY go weird and wavy.
For ONE project such as a barn, if you have enough standing timber get someone in with a portable mill, you can probably get all your rough cut milling done on a deal whereby he takes away the same amount he leaves - you get to keep his bark and wane edge too.