Under 100,000 miles on a car like that isn't bad at all especially on a 98 328i. How much is she selling it for? Is it a special price because they know you?
Transmission should be okay unless you put an unusual amount of wear and tear on it.
Check if the A/C compressor and the water pump have been replaced yet. Look for wear around the front and rear shock towers. And make sure the radiator is in good shape. Test drive it, check everything to make sure it all works, drive around for a bit and lisen for rattles or unusual car nosies or shaking. Other than that it should be okay!
Welcome to Ub.c, Hope to see pictures and posts from you in the future if you decide to buy!