I can add to this posting that I haved worked with IPNetRouter to connect a Macintosh Plus to the Internet. I use a Quadra as (software) router. You can download it and use it for free for a month:
http://www.sustworks.com/site/prod_ipr_overview.html
http://www.sustworks.com/site/downloads_classic.html
Most simple is to use a more modern macintosh like a Quadra with Ethernet and LocalTalk on board.
Connect your Mac Plus with LocalTalk to your more modern Macintosh. Connect the modern Macintosh to the Internet via Ethernet like it normal is.
Set your Apple Talk Control Panel on Printer port. Close and safe.
Start upIPNetRouter and follow the instructions in the manual on page 38, MacIP over LocalTalk
ftp://sustworks.com/pdf/ipnr_manual.pdf
Remember that the LocalTalk network needs a different IP range then your Ethernet range you use for your normal network connections. Example:
Your Quadra has IP adres 192.168.1.10 netmask 255.255.255.0
Use for your LocalTalk part 192.168.2.1 netmask 255.255.255.0
If you give your Plus the IP adres 192.168.2.2 netmask 255.255.255.0 you can connect to your Quadra and to the Internet.
An other option I used is a DaynaPORT SCSI/Link 3. It seems you can buy them at this address:
http://www.retrotechnology.com/herbs_stuff/m_net.html#scsi
I have it running with System 7.0 and an external hard drive. With System 6.0.8 it crashed all the time.
With the DaynaPORT you have a direct connection to the network and it can be used as a normal ethernet card.