Agreed with Retro here. You will find no "killer apps" in the classic Amiga platform, even if you own a PPC accelerator.
I consider Deluxe Paint IV to be a killer app, but that alone would not justify me switching completely, since most of the work I do has to be done in Mac or PC environments (for they do have the "killer apps" I need for my work).
If you tell us specifically what you are looking for with the Amiga we can help you further, but Retroman has given a pretty damn good generic guideline for anyone trying to embark into the Amiga with the intention of ditching the pc/mac in the way.
As for modems and software, you will be catered for in these areas. I remember using a standard Supra Fax/Modem 33.6 for my internet duties, along with Miami, and all that worked well... On my A1200 with a 68030@50 and no FPU. I could browse the EAB at 64 colors, using AWeb-II, at the same speed I browsed the EAB when I had my Pentium 133Mhz.