You mentioned using MOCA so I assume there is coax to where you want to go. In this case if you can still run a moca bridge, you would just need 2 of them, one downstairs and one upstairs. However since its an older house there is a chance the wiring is too old to support it properly.
Since you mentioned the Surfboard assuming it is a model still on the
approved list I believe you can get your own internet account (roommate account). This may benefit both of you since they may be subscribed to a tier that is below your expectations and have an unreasonably low data cap. Again, in an older house the existing coax may not be up to supporting the 2nd modem but more than likely is. I would temporarily try the existing modem upstairs to check its signals.
I've been patiently waiting for moca 2.0 bridges to emerge prior to purchasing more equipment but I would also be interested in getting the Cisco Model DPC3827 8x4 DOCSIS 3.0 Wireless Residential Gateway with MoCA as it would be perfect for one of my properties (even though it is just moca 1.1) but since its not on the approved list I don't think cox will provision it even if I were to find one.