I wouldn't suggest buying the bike if you couldn't check out the bottom end. Rod movements, etc. He could be just selling you a bike requiring a full engine rebuild, which is very expensive. Sleeves are 100$, 150$ for installation. 250$ right theyre, plus the piston kit and gaskets. Another 150$, and the labour would be around 100$.
Now if the bottom end is toast, your looking at a rod kit, which goes for $100 - $150, bearings + seals.. Around $50 - $100. Labour can be expensive, depends whether it needs seals, and bearings or the whole thing.
Wouldn't pay anymore than 750$ CAD.