I don't think there's to many people would say the TD4 is better built than the L Series - I don't think too many L Series engines suffered broken crank shafts or delaminated pulleys etc.. (which are stupidly expensive to fix - if not terminal for the engine) The TD4 does give more refined power and will get away from the lights quicker, but MPG differences are negligible between the 2 engines and overall performance is fine.
The engine is derived from the Maestro engine, and when the Maestros rusted away a lot of the engines went into Series trucks. Perkins did a great job with the (Maestro's) Prima which (Land) Rover then developed into the L Series and then subsequently the TD5 - shame BMW ownership cancelled the Rover TD4 and TD6. You just need to make sure the fan belt tensioner is OK on the L Series.
As I say, the TD4 isn't a bad engine - but it has its faults - just because its made by "BMW" means sweet FA.
Parts availability for L Series is fine. There's always problems getting parts for Freelanders over here, but at least with the L Series engine I can get bits from Rover shops. Everything else is ex UK and readily available at decent prices (before shipping!).
I think choices between L Series & TD4 have nothing to do with the engine itself. If you want a car with more 'luxury items' that only went into later cars then you are forced into TD4. If you want an auto, then you are forced into TD4. If you want the facelift, then you are forced into TD4. If you are happy with an older, more basic manual car, then the L Series ones are the way to go because they will cost less overall to run (or at least less likely to require fixes that cost $$$) and be more reliable.
