Normal towing (uphill, downhill, flat, dirt/gravel/grass and the manual has no trouble with 1800kg. Your ball weight ought to be around the 160-180kg mark, and with a bit of distance between the ball and the rear axle, the WDH is a good idea - but try it first. take it to a dirt road and try steering/braking. If your steering and braking aren't as good, a properly set-up WDH will fix it. I tow (auto box) a 2550kg caravan and always use a WDH.
However, if you need to use a boat ramp, or a steep driveway, your clutch might suffer a little. The D40 manual is geared a little high, and slow-speed manuevers require a little feathering of the clutch which is ok when there's no great load on it, but on a steep incline it can cause the clutch to age very quickly. This is because the flywheel is a dual-mass flywheel which has a thin face, which doesn't allow heat to dissipate quickly enough, so the clutch face burns.
If you can stay away from those, the D40 manual has no trouble at all. 1800kg is an easy tow.
Just watch your turbocharger. If you're towing, your turbo works a fair bit harder and needs to cool down a little longer when you're done.
I haven’t towed a lot with my D40 manual, but when I have I’ve notice of that it does take a bit more effort to get moving as expected. 1st gear is a little high, as stated, and it can feel really slow off the mark until the turbo boost kicks in and takes a lot more clutch slipping than non towing. Once moving it tows well. I barely towed anything with the stock clutch and now have a single mass aftermarket one - and it hasn’t done enough towing to comment on longevity.
Having low range available is great for steep boat ramps etc, just need to be careful about turning too much. I’ve used the Nav to reverse our small van into a tight spot and once just about lined up I’ll drop it into low range to crawl in the last bit. Was good to be able to drag a loaded car trailer out a steep, narrow but straight driveway recentlt too
Mines a D22, so not the same, but even more challenged when taking off while towing; I tow for work everyday. More often than not, with a lot more than 1,800kg in the trailer.
On flat ground I just let the clutch out & it idles off the mark without stalling, even with big loads in the trailer. I don't touch the throttle until the clutch is fully out. It's slow, but it works.
On steep inclines, I have to slip the clutch when towing above about 2,000kg to get moving. I've stalled it twice with a HUGE load in the trailer, trying to take off on an incline.
I found that the ball weight affects mine far more that the actual weight being towed does. My work trailer is dual axle & only ever has about 60kg on the ball when loaded correctly.
Our camper trailer maxes the ball weight out, even though the camper only weighs less than 750kg fully loaded.
It's much easier to tow 2,800kg in the work trailer with 60kg on the ball, than it is to tow 750kg of camper trailer with 180kg on the ball.
Just my experience & with a D22, so not quite the same, but similar.
Ball weight is absolutely critical on a single axle van. Having just one set of wheels means there's no extra friction to overcome to turn the trailer (try manhandling a double axle trailer on flat concrete one day - in a straight line, yep, but try turning it - nope!). The single axle trailer will happily sway and wiggle behind your car from various provocations like poor loading (far less than the suggested 10% trailer mass on the ball), wind, steer wobble (eg going around a pothole), potholes, errant braking, turning while braking etc etc.
Double axle trailers have an advantage there. Almost everything will follow the path of least resistance - including your trailer. Because the wheels resist turning, as the trailer tries to start wobbling the tyres bite into the road slowing the trailer for you. It doesn't prevent sway - but it does reduce it. Because of this, and that there's two axles to support the weight, you can reduce the ball weight somewhat - but this doesn't work with high loads (where the weight is high above the axles) or large loads far from the axles (like putting a huge generator in a box on the back of the caravan).
The best thing to do is follow the guidelines (single axle = 10%, double axle between 5% and 10%) and see how it behaves.
I'm with you all he way on trying to move a dual axle trailer around on concrete when loaded Tony. I just did so this week on a job site & it was a nightmare! So much so, that I'm thinking of taking my ratchet moving jockey wheel from the camper to work with me in future.
Our last camper trailer was single axle soft floor with about 48kg on the ball.
A few of the places we took it were; Oodnadatta Track, Mt Dare, Dalhousie, Chambers Pillar, Finke, Old Ghan Track, Lambert's Centre, Painted Desert, Mereenie Loop, Eringa Water Hole, Charlotte Waters, Palm Valley, Cameron Corner, Dig Tree, Eromanga, Quilpie, Thargo, Eulo, Dowling Track, Strez Track, Birdsville, Birdsville Track, Flinders Ranges many times, Gawler Ranges, Stuart Hwy a few times, Savannah Way through Hells Gate, Roper Bar to Borroloola, Lorella Springs, Burketown to Adelaide via Winton, Longreach, Charleville & Bourke.
I'm sure I've forgotten some of the places we took it, but we had a ball.
As an aside, it was Chinese. And it never let us down, anywhere!