Visit
Disco Web for all your Disco information. FYI - don't post another "Should I buy..." thread there. Instead spend a morning cup of coffee reading what's already there. Just like Pelican the search function will uncover any/all info you need.
Pre 1999 Disco were series I. They carried a locking center differential (LCD) stock and were easier to work on.
1999-2004 was the Disco II. Some early 1999's and all 2004's had a LCD. The others got by with LR's Enhanced Traction Control (ETC) which used the ABS modulator to limit power to slipping wheels. Works great when it works... Disco II has known issues with ABS lovingly referred to as "The Three Amigos" (for the warning lights in the dash) which can be very spendy to fix. The Disco II uses Bosch (vs Lucas) electronics and overall is much nicer equipped and finished than the series I.
LR is not known for reliable vehicles. It seems though that the harder you beat on them the better they do. My wife started with a '97 series I, then moved to a 99 series II, and now drives a '03 series II. We love them but readily admit they are a love/hate relationship. If you really like them then the rig you linked sounds like a good deal. Definitely have a PPI first and the more service history on it the better.