Check for blowby (idle, remove breather & shine a bright flashight like a mag lite or mini mag lite or LED flashlite on gas venting out, you shouldn't see a mist of oil) or look in the air cleaner, which should not be slimed with blowby, look for oil leaks. Ask about fluid use, pass on anything over a quart a month (you know they 'round down') & service. Since SVO conversion can be done quite incorrectly, I would pass on a car with 'single tank' VO use or high percentage (>15-20% SVO) blending unless it was for parts. No matter. Some chump is probably driving it to lovecraft biofuels right now.

Worst case, if the engines a stinker, you could pull lights, dash, trim tranny & rear end, scrap the body, & sell the parts.