I have a 200TDi, but they're not that much different - the 300TDi is a little more refined, but most of the LR fraternity seem to prefer the reliability of the 200TDi.
New 300TDi conversion kits (engine plus all ancillaries) were available from someone in the UK for £3200 or so a short while back.
Assuming you have a N American Discovery, the 300TDi would probably be the one to go for, as the introduction of the 300TDi version in Europe coincided with the introduction of the Discovery over here in 1994/5. In other words, what we know in Europe as the 300TDi had the bodystyle of the first N American versions. My 200TDi has the earlier dashboard & interior.
- Richard
PS - The $h1tty diesel over here doesn't seem to be a problem with mine - I just add the same dose of Millers Dieselclean Plus as I did in the UK, but here it has a significant Cetane Boost characteristic.