For your type of business, I would have thought that some sort of AWD capability essential, particularly as your towing trailers off road and negotiating steep a drive way at home. Just wondering why you want to change from a defender which has a much greater towing capacity and other attributes.
If you want a T6 or T5 then 4Motion would be essential and one with a rear differential lock even better. The rear Diff Lock does more than add additional traction. It pre-engages the 4Motion system so the vehicle feel so much more positive off road because there is no pause for activation and so less chance of initial slippage. This may be important to you in that repeated visits to a clients wet and slippery grassland there is less likelihood of turning it into a quagmire. All Terrain Tyres would probably be a good idea. Equipped as such a T5 or T6 would provide excellent traction. Most Defenders only come with a central Differential Lock the axles being open, they do however have a low range for creeping but a T6 or 5 with DSG can be encouraged to drive slower with the brake pedal. DSG equipped vehicles do not normally stall. There are outlets that can put in a lower first gear. I live off-road so my T6 4Motion Caravelle Exec with rear Diff Lock gets used almost every day over Welsh Green lane conditions including sloping wet grass. It does very well off -road only lacking in massive articulation and ramp over capabilities of a defender otherwise it will follow a Defender quite well under axle clearance is also quite good. T6 will take larger tyres plenty of information on these pages.
I am not a mechanic but I believe that a T6 properly looked after ought to provide good service so any used vehicle needs history and a good look at. The Haldex oil needs changing every now and then, mine was filthy dirty at 27,000mls. If no record of it being done the have it changed and cleaned out. I think that VW does not mention it or perhaps expects the oil to last the vehicle life time, it wont! No idea of minium price but Defenders are not cheap either. Find a good VW Independent Workshop for servicing unless your good at that sort of thing.
There is massive information on these pages and some very cleaver and experience master mechanics and electronics people here, they have dazzling knowledge and capabilities. Good luck with your search and decisions.