Hello,
Nikasil is potentially a very serious issue. The easiest way to tell if a car still has a Nikasil block fitted is to ring BMW UK with the chassis number and ask if the engine block was changed under warranty. The telephone number can be found on this site somewhere but you can also get it by ringing your local BMW dealership.
There are people on this forum who have Nikasil engined cars with no problems but I personally wouldn't take the risk. If you do get a Nikasil engined car and it does have problems it will cost you a fortune to fix.
Ahhh...
Just noticed you said S reg. This car is probabaly not affected since S reg is late 1998 to early 1999. And BMW didn't make any more nikasil engined cars after early 1998, although a few may have been made then but not registered until later.