G'day Andrew,
Mustang's were a pretty good boat. The hull will be fine, the floor and transom were the main problem areas for boats of this vintage. Any soft or springy areas should be checked for dry rot. A lot of boats of this vintage were not glassed under the floor where you could not see and consequently rotted out slowly. If the transom has been repaired I would check if it has been done professionally. The old V4 OMC's were a pretty reliable motor. Knock him down a bit (if they say "around" usually they will take a bit less. I would take it for a run first to check how the motor runs under load as running it with the muffs doesn't really give a good indication of how it will perform under load.
Cheers......Brian
1985 110 V8 County
1998 110 Perentie GS Cargo 6X6 ARN 202516 (Brutus)
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