You didn't mention the SMOH, but you're asking about overhaul costs,
so maybe it's getting close to TBO?

$33000 sounds like a high valuation(based on 6 x $5500) for a 1969 140
with original paint. avionics - are the nav/coms digital or analog?
any GPS? how's the interior?

You might take a look at Trade A Plane and aso.com for comparable
valuations to see if $5500 is really worth investing in something
that's worth less.