Just to clarify a few things, most non-compact, non-laptop machines from the Mac II and newer can support multiple network cards. Many have a motherboard network interface, plus you can add at least one more via NuBus or PDS. The exceptions would be the LC models, the Quadra 605, the IIsi, and perhaps a few others.
With regards to drive sizes, any machine with a SCSI bus can support drives up to 2 terabytes. This is because all SCSI block numbers are expressed as 32 bit numbers, so you can have up to 2^32, or 4 billion, 512 byte sectors. Of course, there are various limits on volume sizes, but you can use any size disk on SCSI busses. I have 250 gig and 500 gig drives on SCSI-IDE and SCSI-SATA adapters on m68k Macs, and I even have a 1.5 terabyte drive on an Amiga 4000 (SCSI-SATA). IDE, on the other hand, is limited to 128 gigs (2^37) on m68k machines with IDE (some people call it 137 gigs because it's 137438953472 bytes).
I've used m68030 and m68040 type machines to do NAT, DNS, IPv6, port forwarding, and so on. A typical m68030 machine like an IIci can do at least 100 kilobytes/sec and usually more than 150 kilobytes/sec, which is around the speed of a T1. This is perfectly adequate for slow broadband such as phone company DSL. An m68040 machine can usually do around 300 - 400 kilobytes/sec, which is good for slower cable modems or faster DSL. I even had five ethernet cards plus the motherboard ethernet in a Quadra 950 back in the day, which was used to share internet with three different buildings. Macs, in general, are very stable machines.
I run NetBSD on my machines because there is no standard GNU/Linux for m68k, plus compiling GNU/Linux for yourself on a legacy architecture can be a full time job in and of itself. NetBSD, on the other hand, supports older machines just fine, and the source tree will build m68k stuff simply and easily without fuss on any other machine (for speed), or natively, if you have the patience.
We (NetBSD) also have a binary distribution so that people can make use of LC040 systems, which isn't possible with GNU/Linux or the standard distribution of NetBSD (well, it works technically, but the system will segfault like crazy). So if you have an older machine, buy a NuBus or comm slot ethernet card, install NetBSD, set up NAT, and you're good.