Never go and pick up an aircraft in one day! For 139 acceptances we had a 45 page document where we would test EVERYTHING in flight and would stay till everything was fixed. If you leave, they move on to work on that machine of someone who is still there putting pressure on them to fix things.
SK was the same with picking up 92's. You have to be there and put the pressure on. If you're not getting good results, time to get money off the purchase! Make sure you have firm clauses in your sale and purchase agreement about compensation for delivery delays that are not your fault.