Defender was basically the name that LR gave to the 90/110/130 in about 1990 ish, make it sound more modern.
As for getting it running, why not crack off the injector pipes at the injectors, use the lift pump to prime the system, then retighten them and try to start it off a very well charged battery, it should fire up.
Hammerite should be ok in the engine bay, you may get some problems by the exhaust manifold with heat, although you could use high temp paint down there.
TBH though, i wouldn't worry too much about draining the oil out of the old engine anyway, as it's coming out. Wait until the new one is in before doing the axles/box etc, at least you can get them warm on a test run then.
You say it's on the garden? How are you planning to get the old lump out, and the new one in? As you are going to need a hoist of some description.
Sound deadening is personal choice really, but i would think along the bulkhead is the best place, along with the underside of the bonnet. You could always do the inside of the bulkhead, under the floor matting, although if it gets wet, then you will effectively have a soggy sponge rotting the floors from the inside. Spray on underbody shutz has some sound proofing properties, so it could be worth spraying the underside of the cab area with that, whilst it's dry.
There is an excellent guide in the tech archive to converting to 200tdi power, well worth reading. Some of the steps can be altered to suit.