Reddit mentions of Juniper Srx-mp-1docsis3 1-port Docsis 3.0 Mini-physical Interface Module

Your modem does not need support for more then one ip for you to have multiple ips.

As for rackmounted DOCSIS equiment, I hope you have deep pockets and don't mind a slow 4x4 modem.

https://www.amazon.com/Juniper-SRX240H2-Series-Services-Gateway/dp/B00C1YMQCS/ref=sr_1_1?ie=UTF8&qid=1428086852&sr=8-1&keywords=juniper+srx240

https://www.amazon.com/Juniper-Srx-mp-1docsis3-1-port-Mini-physical-Interface/dp/B00454U5PW/ref=sr_1_fkmr0_1?ie=UTF8&qid=1428086900&sr=8-1-fkmr0&keywords=juniper+srx+docsis+card

Also, for a router, no, that $68 router is not going to suffice for usable wifi for 300 people.

Try more like this:

http://www.amazon.com/Juniper-Srx-mp-1docsis3-1-port-Mini-physical-Interface/dp/B00454U5PW/ref=sr_1_fkmr0_1?ie=UTF8&qid=1428086900&sr=8-1-fkmr0&keywords=juniper+srx+docsis+card

with one of these: http://www.amazon.com/Juniper-SRX240H2-Series-Services-Gateway/dp/B00C1YMQCS/ref=sr_1_1?ie=UTF8&qid=1428086852&sr=8-1&keywords=juniper+srx240

The SRX240 is a Juniper router which can accept a docsis interface and has various ports on it.

All you'd need would be the SRX240, the Docsis interface card for it, the XR-4000, and the PoE injector for the XR-4000 (it uses 60 watt PoE on 1 of the 2 cables).