Need a 524td buyers check list.....

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I have never owned a diesel car before. I am not very familiar with the e28. I only had one a while ago, but i didn't drive it. I bought it to part out. I have owned some mid 80's BMWs, like an e24 635csi and a e30 325i. there is a decent 1985 524td in my neighborhood that is for sale. this sparked my interest b/c of gasoline prices being so high, it might be cost effective to run veggie oil or some sort of biodiesel. ANYWAY, The odometer shows 89K miles and the guy is asking $1600. assuming the worst case scenario, what are the most important things to look out for when buying a 524td. #1 being the most important. Please make me a list. Thank You