If that is a Giradin body then it is mainly aluminum. very lightweight.
I have a 1997 e350 5 window thomas and a 2002 E350 5 window as well.
The chassis of the e350 and e450 are the same, The e450 have dually rear wheels and can take a little more weight. We use our with 2 kids and a dog , no bathroom, and are fine, The e350 single wheel are just wide enough to sleep in transverse direction, with me being 6ft02". The girardin is slightly wider.
If you build sensibly then I do not see a problem.. with the lower weight and less width it is easier to drive, park, faster, stops faster, better fuel economy and less wear on motor and tranny.
the 240 Kilometer if kilometer aznd not miles is not bad but not great either,
Check if the injectors were replaced and blowby, let them do a cylinder contribution test.
For the rest check for rust.