I am not buying a modem for this server. If I were to connect my home computer to this server via ethernet and developed my Zope site on this server, while in development could ZServer access the internet (provided I am currently connected) through my Mac's modem?

AFAIK, only if your Mac was running some sort of Network Address Translation (NAT) software. Linux does this rather well (embarassingly, when we moved offices recently the whole LAN was dialed into a single ISP account at something like 44kbps using RH5.2). I think there is a NAT How-To in the Linux Doc Project... I think... --Rob