Very little difference between a 200 and 300Tdi. They are essentially the same engine. 300Tdi is claimed more refined, but in truth I can hardly tell any difference.
Some late 300's (I think maybe autos??) had an ECU whereas all other Tdi's used a manual injector pump. The only time this'll be an issue is if you want to tune it/tweak it.
There are other differences between the 200 and 300 Series Disco's.
-Gearboxes. 300 has the R380 and 200 the LT-77. They are essentially the same gearbox though. Many claims the R380 is better, but I've certainly driven a share of worse ones too.
-Axles. 200's are like an old Range Rover underneath and use the 10 spline axles and diffs. 300's I think swapped to the 24 spline shafts and diffs. In reality there is very little difference in outright strength, although I think there are more options to beef up the 24 spline ones if you plan an extreme off roader running 35" mud tyres.
-Subtle exterior trim. 300 has different grill, front lights, front indicators, door handles and bigger mirrors. Also rear lights in the rear bumper. None are really any different or superior to a 200 which uses RRC/Marina/TR7 door handles and RRC door mirrors.
-Interior. 300 Series has a very different interior, different dash, seats, trim the lot. Personally I think its a MASSIVE backwards step from the rather funky and open original Discovery.
That's it mostly. Personally I think a 200 Series stands a fair chance of being in equal condition and is just as capable with a nicer interior.