You've probably already done some research but the 325iS is still pulling big money and as you say they are fairly rare. The very nature of them also means that they have probably been driven 'spiritedly' over their lifetime. One of the attractions of the E34 for me (my list is infinite!!!) was that the 5 is what I call a 'grandpa's car' and was usually bought by mature people who don't usually flog them and have had the money to throw at them when they needed it. Mine is a classic example of what I'm talking about.
Cheers
Wingman
'89 525i/A Exec 193000kms
'94 Subaru Liberty AWD Wagon
Looking for Merc 300TE
NSW, Australia