Hmm,
I would say go for the plus, just because your more likely to find suitable BBS software and its easier to add a HD

I know that the old WWIV BBS ran on a 512k (from personal experience) with as little as two 400k floppys - but clearly this is not the optimum configuration

I dont think I ever came across (but am pretty sure there was) any other BBS software that would run on a 512k with 64k roms or less (i'd like to see it tho).
You mention you'd like to have both dialup and internet telnet clients - given this, I'd guess you would need a BBS package that supported simultaneous multi-line connections - I say this just because I cant think of an easy way to share a single serial port with a standard modem and an internet telnet device at the same time. If I remember correctly, the Hermes BBS software did support two concurrent connections via devices connected to both the serial and printer ports (and ran on a plus). In your case, one of these could be a standard modem, and the other could be a "ethernet modem" or PC acting as an ethernet modem.
Ethernet modems essentially look exactly like a hayes compatible modem to the serial attached device, but connect to ethernet networks via TCPIP and allow other TCPIP attached devices to connect to the serial device (via telnet etc) - the serial device on the end doesnt know the difference as the ethernet modem handles all the "ringing/hangup/carrier/handshaking" signals based on what connections it is accepting/processing via TCPIP. BTW, this is a good way to get a 512k (or eariler) "onto the net" with MacTerminal or similar - you can (theoretically - I've never tried it on an old mac, but have on other devices) "dialout" to a internet based telnet server via a command like ATDT - cool.
I've used a MOXA single port device in the past and found it quite good (
http://www.moxa.com/product/NPort_DE-311.htm). Also quite good is this software (
http://boycot.no-ip.com/InternetModem) which does a similar thing but runs on a windows based PC.
The only thing I would say about the ethernet modem route is that getting file transfers to work may be a challenge.
Anyway, I'd be interested to hear how you get on, and what other information you find out in the process.
Cheers,