I would insist that the engine be absolutely cold when I arrive. It has been my experience that worn diesels always start better when they have been warmed through. It is that first start in the morning that tells all.
I would also sail the boat hard and look for weaknesses in the rig. There is nothing quite like a rail under to put the seller of a weak boat on edge.
There is a tide in the affairs of men,
Which, taken at the flood, leads on to fortune;
Omitted, all the voyage of their life
Is bound in shallows and in miseries.
Shakespeare, Julius Caesar IV, iii, 217