The answer to this I think is keep it simple. A max weight tractor unit is going to have all sorts of bells and whistles you just don’t need let alone want the attached aggravation. Even with a body the handling/ ride is not going to be pleasant. You’ll have low mounted front spoilers and cab steps to knock off on forest tracks and the already mentioned traction problems. Just because you can read claims of 11/12 mpg with a light load does not mean that removing the trailer is going to dramatically improve that figure.
Go smaller, go run of the mill. It used to be said that the working man’s car was a Ford and the tradesman’s van was a Transit. Now it may not be quite the same today but the principle does still just hold. No matter where in the world you go you’ll find someone who knows how to fix it. To transpose that to heavier commercial vehicles we need to look for the most popular driveline components in the 7.5 - 18tonne sector, which leaves us with two choices DAF and Iveco both of whom use their own version of the ■■■■■■■ ISBE engine, badged as Paccar or Tector. Both manufacturers also have very extensive dealer networks and their bread and butter products form a substantial proportion of the workload for the independent heavy commercial vehicle workshops around Europe. Behind that engine series we need to have a simple manual six speed gearbox. Now we need to decide on actual design gross weight 7.5t, around 12t or 18t. The higher up the range we go the greater the ground clearance - in theory. A probable compromise between that and fuel consumption rests around the 10/12t mark. You won’t achieve anything brilliant, particularly in low gear up forest tracks, but 12/13 mpg or better should be possible on tarmac.