It is was an older 350, water in the oil wouldn't worry me. A 350C has three seals per sleeve and it's rare to leak coolant into the oil.
$6000 is not cheap and I'd want to know what's wrong with it - or get the price lowered to $4000.
I'd be more worried about the transmission case and reverser case (where they join). Look for any signs of movement there and/or loose bolts/nuts. There are two nuts on top and two bolts on the bottom. A major weak point on Cs and often - no cost effective way to fix. I've seen many go to the scrap-yard when it happens. It's been loose -it breaks side-frames where they bolt to the clutch housing and ruins the output shaft in the reverser and the input shaft in the transmission. I just passed one up with a 6-way blade that was selling for $2500.