If you want to keep it in the desktop - pcmcia network card and a accelerator card (the faster the better)..
About virus on the internet, dont worry

No virus/spam/whatever will work on the Amiga browsers.. So safe browsing..
You dont need os 3.9 to connect top the net, you only need a tcp/ip stack ; MiamiDX or Genesis/AmiTCP (included in OS3.9)..
Seach for these on aminet.net

Good luck, and post your progress..