If it were me, I'd be giving a deposit and saying that you'll buy the van on the condition that an oil sample sent to Blackstone labs comes back with an acceptable report. Blackstone does quick turn around for people in your situation and I'd expect you'd have a result in a day or 2. I'd also be topping up the trans fluid myself with mercon V just to see if that slip is because of fluid level or damage. At 135k, it's sort of at the "beginning of the end" from the trans point of view. You could also get the vin and go to Ford and see if/what's been done on the van. Brake pressure switch should have I believe. What are they asking just out of curiosity?