The 1997 323 never came to our shores so I don't know what transmission it has, or whether it uses synthetic or conventional fluid.
On my '97 328 auto, I replace the fluid every 60k miles, and it shifts much nicer with a fresh oil change. Even with fresh fluid, however, this transmission tends to hesitate and then shift with a thud when it is cold, and smooth out when warm.
The kit for mine includes 2 gaskets, a filter, and 28 new pan bolts and costs $30 US if you buy Meistersatz brand on line or $180 from the dealer for BMW branded parts.
If yours uses synthetic fluid, you should be able to push that change interval to 100k miles. Good thing, synthetic tranny fluid is extremely expensive.