If it is *truly* piston slap, it goes almost completely away when the motor is warmed up and the pistons expand. If you've never heard it before it usually sounds a bit like a diesel when its cold. Sometimes these motors just keep running like this, and sometimes the skirts on the pistons shatter.
In Mass that plow setup would probably fetch you between 500 to 1k, on the higher end if you do a nice removal with all of the push plates, wiring, headlight isolation modules, etc so its a full package.
Being a northeast guy, the rust in the pics you showed isn't horrible, Also, I don't see any rust on the front fenders (yet) which is amazing. Now, since the frame has already been welded, we can make an assumption that the frame is (or at least was) in poor shape.
For me, I won't buy a TJ with a rusted or even a repaired frame, and 6500 seems very high for a questionable motor and repaired frame. If this one had a non-repaired and not rotted frame, i'd take a very close look at it. Even in this case I'd want to be in it for about 4500 with the plow.
Do yourself a favor and pass. Save some money and keep looking for one that didn't come from the rust belt in your area, or travel (or have shipped) a clean one from outside the rust belt.
That's my $.02, and worth exactly what you paid for it!