For an introduction to Landrovers, I would not recommend a P38a. While in amny respects they are a superb car, they are quite cheap for good reason. While not particularly prone to problems, they do have problems, and these can be very, very, difficult and expensive to fix. Suitable only for owners with deep pockets or a cavalier attitude to risk. They also suffer from the 'luxury car' syndrome - the assumption that if you can afford one (new) you won't worry what we charge for parts/maintenance. And you will buy another one in a few years, so long term support is not needed.
Defender 90s in any price range are quite rare in Australia - the most popular size is the 110, so this will be the most commonly available. As indicated above, 200,000, properly maintained, is not a problem. 
There have been four engines fitted to the Defender. 
The first, the Tdi200 is getting a bit elderly now, and some parts are getting to be a bit hard to find. 
The Tdi300 is considered by many to be the pick of the Defender engines, having minimum electronics. Either of these must have the timing belt changed at the recommended interval - failure to do so can be very expensive.  Both are developments of the original Rover diesel introduced in 1956.
The Td5 is very durable, and more civilised than the earlier engines. It is usually trouble free, but there are two generic problems - some had an improperly installed oil pump bolt, which may cause catastrophic failure. Virtually all of these should have been fixed by now. They also are known for oil ingress into the injector harness, causing poor running - not particularly expensive to fix. The only real caution with these engines is that unlike earlier engines, they do not stand overheating, which will result in very expensive repairs.
The Tdci engine currently fitted is a Ford engine, and it is perhaps significant that there has been virtually nothing written about problems with this engine.
Performance with any of the Defender engines is virtually the same.
From a cost perspective, the best entry into the Landrover world is probably the Discovery 1 or 2. These share the same engines as the Defender, plus the V8 petrol. The Discovery 2 is probably a better and more modern design in most respects, but unlike the D1 (which shares many parts with the Rangerover Classic) parts can be expensive - for example the D1 has replaceable wheelbearings at well under $100 a corner, where the D2 requires the whole hub to be replaced at $350-$600. But the newest Discovery 1 is fourteen years old! 
Best bang for your buck would be a Discovery 1 V8 - you can get good examples very cheaply, and maintenance is not high. But the fuel bills are!
Hope this helps.
John
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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