Who has the most miles on their Transit with the 3.7L V6 and would you buy again? ...

I have just under 12K on mine. The 3.7L is plenty powerful enough to move my cargo. Payload ranges from 350 lb to over 4000 lb on occasions. I kinda wish I had the 4.10 rear axle, but I like the transmission a lot. I'm sure the Diesel and EcoBoost are a hoot to drive, but I don't consider 275 hp to be slouchy. The engine needs more revs than the others, but the powerband is quite flexible. With the kind of cargo I haul, much more power would just throw stuff all over the truck.

here's a video https://vid.me/oJGC

The few times I have weighed my van it was between 8400-9600 lbs, and it never made me feel inadequate for the job. Granted I have to corner quite a bit slower, but acceleration up a 6% grade with 4000 lbs in the back was possible.