My uncle is thinking about selling his 1990 300zx. My question is it has been garage kept for the last 8 years....would it be best to drain all the fluids out before trying to start it?

Yes if its been a while do a full fluid service Oil & filter, Radiator flush, Trans fluid & filter if auto and Fuel Filter and brake fluid ect. Air filter and check rubber items like hoses, vacuum lines, belts, it nothing was put in gas take to keep it in good shape I would take it down and inspect it and possible go ahead and replace it (pump). and anything else I did not think of. May pull thermostat or keep a check on it after starting, make sure cooling fans are working.