As the title says basically. Is it all pretty straightforward?
Maneuver towbar in place and bolt it in with new bolts or do I need to undo existing things and bolt it in alongside them?

The devil is in the detail.

Go to a self service wreckers, and find your model with an intact towbar on it.

Pull it and the wiring off yourself. Look how the wiring runs and where it is connected. Keep every bolt you remove, and note where different length ones go. You will learn everything you need to know while doing the removal.

(Usually the bar mounts to threaded holes that are already there for the bumper bar mounts, and you just use longer bolts, and the wiring runs through a grommet already in the floor and connects to one of the tail lights or is spliced into existing wires.)

As the title says basically. Is it all pretty straightforward?
Maneuver towbar in place and bolt it in with new bolts or do I need to undo existing things and bolt it in alongside them?

If I were you, unless you are absolutely 120% conversant with electrical wiring, I would have the wiring harness fitted by an auto electrician or licenced accessory fitter.

However, if you are going to have a go yourself, this may help:

http://www.narva.com.au/products/browse/wiring-diagrams

who can also supply the correct harness.

I've called wreckers and there are none with a towbar here so I can't go and inspect. Have found someone who has a new towbar for sale but its off the vehicle

I'm good with wiring, I often cut and recrimp new waterproof connectors in place where the plug is either garbage or I simply need to run new wire. I do the job properly
I can't remember if there was a plug in the boot of the car already

But I want to know about the mechanical fitment so I don't find I have to remove half the rear end to fit it. I'm not too concerned with bolts I can just get some high tensile bolts from a fasteners store of the appropriate length. But if dimensions were available that'd be even better, saves me measuring

I think it depends on the type of towbar. I fitted a genuine one to my TH a few years ago and ended up having to remove the rear bumper. It's not difficult just time consuming. I did read a couple of threads here that it is possible to get away with not removing it but I found it was easier just to remove the bumper.

In terms of electrics I just bought a harness from here http://www.towbarsaustralia.com.au/. It's pretty much plug and play, just goes in line with the rear lighting run in the boot.

So I had a look at a towbar for sale privately, it's missing the tongue. It's a Hayman Reese P/N 1519
According to their website this is the correct towbar
Can't call them over the weekend. Anyone have any further info as to whether or not I can get the tongue?

How much do they want for that?
I paid $110 several years ago for a towbar, complete with tongue, bolts, wiring harness.

Best check availability and price of the tongue, and add that to the cost of the towbar - then decide if you want to buy.

The wreckers are the best option (so long as they aren't greedy) The one for my Wagon was asking $100 sold to me for $80 after a haggle and included the wiring harness which simply plugs into the existing loom and all the bolts for the holes are already there.

I paid $50 for my hayman reese towbar for my verada from the wreckers. Had to remove the rear bar to fit it. I also used flat 7 core trailer wiring so to keep it in the boot when not towing. All the wires to splice into are on the right side tail light harness by memory (correct me if im wrong)

No splicing needed on the TJ bar I installed. It plugged into the passenger side tail light, with the tail light plug going into the rear of the towbar plug. I didn't remove the towbar or wiring off the original car, so had to experiment with a number of possible setups until I found the right one.

Hence my suggestion to try and buy one off an identical car at the wreckers and remove it yourself.

How much wear and tear does towing put on the transmission, seeing as most cars don't have ATF coolers?

And does it do damage to the chassis? I heard someone say (but this kind of sounded a bit off) that if towing heavy loads it can somehow warp/stretch the chassis?

How much wear and tear does towing put on the transmission, seeing as most cars don't have ATF coolers?

And does it do damage to the chassis? I heard someone say (but this kind of sounded a bit off) that if towing heavy loads it can somehow warp/stretch the chassis?

Just don't stress the gearbox, change manually down a gear at the appropriate time so you use engine revs rather than rely on the torque converter slipping a lot to maintain speed.
Accelerate gently and perhaps shift manually at a higher rpm than you would normally.
Remember to look further ahead once moving and allow for longer braking distances.

I've also heard someone (smarter) say that if you stick to the correct load ratings for the towbar and car, nothing will stretch, self destruct or fall off. lol

Stick another Magna on a car trailer and tow it with your Magna, anything can happen.

I highly doubt anything would happen if towing up to the rated capacity

The towing capacity of 1500kg is not 'just barely 1500kg on a dead flat surface accelerating at a snails pace'. It's 1500kg at highway speeds and under expected conditions, which is some modest hills, typical acceration rates etc where the effective load could spike up to 2000kg or more periodically. If you started drifting a 1500kg load that's a different story, but I would have a hard time believing anything would be damaged if you were to tow say 500kg above capacity but compensate accordingly with very smooth roads, very slow acceleration and no sudden weight shift

That said, i'd be interested to see how much the engine would happily cope with. 1500kg seems rather low to me, I doubt it would break much of a sweat whilst hauling that

The bigger issue for me is the damn downforce rating, it's always piddly in cars. I'd like for it to have a 200kg+ downforce rating so I could use one of these
http://img.auctiva.com/imgdata/1/0/7/5/3/4/6/webimg/270282618_o.jpg

I believe the downforce rating is only something like 90kg?

1500 kg towing, provided it is a braked trailer and you have the heavy duty towbar.
You don't mention that, but say 500 Kg over won't matter much?

2,000 kg on the back of a Magna, and you wonder if the engine can cope? What about the brakes?
How about the tow bar ball? What is that rated at? (shock load, not static load)

Anyway, I think there is a road law that says the trailer and load can't exceed the weight of the car.

One other aspect to consider is how would you cope with super light steering, it would be worse than driving on ice.

From what I understand you are meant to balance the weight on the trailer such that the car sits at the same level a without the trailer so steering would be ok?

I've towed twice in my life, once with a VT Commodore and once with an EF Falcon. Both cars did remarkably well to be honest (each was towing about 1,800kg)