OK, a quick start then:
200Tdi.... in defenders and discoveries, these never had much in the way of immobilisation on them. Some discoveries (and later defenders) had an alarm with immobiliser which just cut the starter and fuel solenoid (behind the dash), nothing actually "on" the engine, so with a length of wire clipped onto the fuel solenoid and reaching over to the starter +ve.... wahey! off ya go.
The discovery and defender 200Tdi's are slightly different. the turbo/manifold assemblies are different, as are the front cover/timing chest.
discovery 200's won't fit "directly" into a 90/110 engine bay,using genuine parts, without a little tweeking. The exhaust downpipe is the main culprit. but a few hours welding with some pipe can sort that, or the likes of Steve Parker
make off the shelf converted pipes. In fact, if you look at his page you'll see all the little kits he does to make lifes easier, although it's only pipes and stuff, so nothing that can't be fabbed locally.
the 200Tdi will bolt straight onto an LT77 gearbox that used to be bolted to a 2.5 diesel/petrol (that's NA diesel and the old turboD) and also, with the removal of a couple of studs and drilling the holes out slightly on the bellhousing, onto a series gearbox.
300Tdi: there wree several types of these.
the early defenders/disco's had a plain engine, no EGR (Exhaust gas recirculation) or immobilised FIP. there was a "spider" alarm unit fitted to later models, but this just cut the starter and solenoid feed (it mounted under the LH batteries in the seatbox, in a sealed box).
Later still, some engines had a "digital immobiliser" fitted to the top of the injection pump, this had 3 wires, but if you break off the plastic cap, you can remove all that junk and fit a normal solenoid with one wire to feed it.
vehicles with EGR also had a "throttle position sensor" on the linkage, this was just to tell the ECU where abouts the throttle pedal was, again, this can be junked, as can the EGR valve (with a suitable blanking plate over the exhaust manifold)
Later discovery auto's had Drive by Wire engines. the basic engine is the same, but the injection pump had electronic feeds to it instead of the older type steel pedal cable, and it's not too cheap to get the pump converted back to non-electronic (not to be stated that it can't
be done, it's just a hassle, so be careful when you buy an engine that it's the correct one.)
LT77's (from 4 cylinder vehicles,) and series gearboxes can also be bolted to a 300Tdi, but you need to remove the flywheel housing from a 200Tdi/2.5 engine and bolt it to the 300 first. See "Nicks Jungle" (a friend of mine, Nick McFarland in Ireland)
who has done this, and various other mods to make a 300Tdi fit.
On this page, he shows how the glowplug wiring is taken care of, using the factory parts
Attached are the mounting positions of a factory 300Tdi in a defender, using an R380 (also mounted in the factory positions), plus the dimensions of the 300Tdi engine mounts themselves.
shout if you need more info.