I am in the process of replacing the 2.25L petrol in my Series III 109 with a mercedes 3.0L diesel. I had the motor from my old 300TD. I found the adapter plate for it on line at
www.seriestrek.com .
There are numerous kits out there for many different diesel engines. They range from a couple of hundred to a couple of thousand.
In my case I have about $1500 in the conversion excluding the motor. I had to buy the adapter plate, fabricate new motor mounts, have the sump modified (welding and fabricating aluminum) put in a remote oil filter and new oil cooler, add electric cooling fans instead of the belt driven one, do some wiring magic for the glow plugs and I replaced the gas tank and fuel lines and I got off pretty light. I am almost ready to fire it up.
What will be interesting is whether I will need an overdrive for highway driving or if the diesel will turn the revs needed to drive over 65. Of course this is a series that I am talking about. A defender would be geared better with a five speed and the 3.54 diff. I may change my ring and pinion gears out depending on the situation.
So to answer your question, I think that it is going to cost you more than a couple of grand if you have to buy the engine.