According to this website
www.motorbiscuit.com
2015 and 2016 should be avoided.
According to this thread
www.fordtransitusaforum.com
2017 is still a problem w water leak
and someone said "Ford "fixed" the airbox issue with a redesigned airbox for 2019."
So seems I shouldn't go older than 2019?

Avoid these Ford Transit Model Years They Have the Worst Problems
The Ford Transit is praised for its versatility and affordability. However, there are a few model years with some problems.

2015 and 2016 should be avoided.
According to this thread
Underhood Water Leaks
I spent some time this afternoon taking a closer look at the sources for water leaks above the air filter box, electrical connections and engine components of the 2017 Transit Van. I have included several photos and a brief description of what I found. Leaks Above Air Filter Box and Electrical...
2017 is still a problem w water leak
and someone said "Ford "fixed" the airbox issue with a redesigned airbox for 2019."
So seems I shouldn't go older than 2019?