After looking at Jeremy Walton's BMW 3 Series Enthusiast's Companion (Bentley Publishing), I found the following changes listed for 1998 on page 339. Editorial comments mine...
Standard for 1998: side airbags in the doors, adjustable rear headrests (which are a p.i.t.a. and require major back-and-forth motion to pull up), HK audio includes CD (which you said sux - why? - I liked mine well enough before replacing with a phatnoise unit).
Optional for 1998: keyless entry standard with sport/premium packs
That's about it, from what I can read. Note that leatherette became standard (vs leather) on the 'vert in 1997. The leather option added wood trim, and in '97 the onboard computer became a stand-alone option.
Now that you've got it, are you going to "play" with it? I've just got the modding bug and swapped to 17" rims (a must - can really tell the diff), added mudflaps (b/c the 17's were tossing up too much grit), and this week added a performance cool-air intake (and can tell the difference). Toying with a chip mod. And trying to find a hardtop...
Have fun with it - I love mine!