I can't help with the towing issue, but here's some advice on buying an E-F 'Defender'
Type: Are you looking at a Ninety (short version) or a One Ten (long version)? If you are after heavy towing duties, the One Ten will be far better. They will go (a bit) slower, but the physics of the 110-inch wheelbase mean you have much more control over any trailed load (the 90-inch is a bit bouncy). One Tens also have self-levelling suspension and a higher rated load capacity due to the stronger chassis. They also get better brakes- which would be important for towing anything on an A-frame. As an added bonus you can carry far more stuff inside and you get better ride comfort than a 90.
I think the rated towing weight of a One Ten is 4 tons with a braked-trailer. The permanant-four-wheel-drive gearbox provides lots of grip for towing, and you may find yourself using the Low Range gears up steep hills!!!
Engine: the 2.5 'normally aspirated' (ie non-turbocharged) diesel engine is a basic 4-cylinder, 8-valve unit based on the 1962-launched 2.25-litre version. These engines are tough and easy to maintain. They don't wear out easily and like many diesels work best under some load. They only develop 68 horsepower, but all the torque is generated at low speeds making them good towing engines.
They should start easily once the glow-plugs have heated up, without too much smoke. A bit of blue smoke when they're cold is fine, but under normal driving there should be none. Black smoke means the injectors need adjusting, whilst blue smoke points to something more serious.
Don't expect to go anywhere fast with one of these engines- 70 mph top speed, with 60 being more realistic.
An E-F reg Land Rover may have significant amounts of rust in the rear crossmember- these are replacable at a price. The chassis outriggers do rust as well, so check these. The top of the bulkhead (around the air vent flaps) rusts as well, but repair sections are available. In about 1985 Land Rover stupidly removed the galvanised body caps, so these rust quickly- but this usually only cosmetic.
Mechanically these vehicles are tough old buses and the only problems are the axle universal joints- which reveal wear by clonking, rumbling noises and the steering swivels which will dribble oil if they are worn.
Hope this helps,
Jack