Just to add, I had a guy come by the house today and he had a later (2001?) E350 with a v10 and it had 300k miles. He said the only issue he has with it is keeping the gas tank full.
The 4R100 behind the V10 is plenty reliable. Much more so than behind the 7.3L. While the closer gear ratios of the 5-speed are preferable, I wouldn't steer away from a 4-speed V10 if it's otherwise in good shape. My 4R100 has 443,000 miles with zero repairs, and it gets worked hard almost all the time.