If the vehicle has a gross plated weight in excess of 3.5 tonnes then you require an operator's licence to carry goods associated with any trade or business. I can't see how you could 'downsize' a 7.5 tonne vehicle by 4 tonnes!
If the vehicle has no gross plated weight, an operator's licence is required if the unladen weight of more than 1525Kg. (I'm also mystified as to how you would seek to reduce the unladen weight of a vehicle).
I haven't got a clue as to what your reference to '6 weekly inspections' is about. To get an operator's licence you need to show that you have arrangements in place (e.g. by a contract with a garage) to keep your vehicle(s) properly maintained, and that you have the financial resources to comply with that commitment, but there seems to be no '6 week' rule in place.
Chris