"Modern" Ethernet over coax

Netgear offer 270Mbps Ethernet-over-coax adapters (MCAB1001) for under $200. They can (apparently) coexist with your cable system (if you have one) and provide a 10/100baseT (RJ45) socket to connect to your LAN/computer.

Not sure what the deal is regarding the advertised 270Mbps transfer speed being limited by the 100bT connection though.

I really think you should go with plan B, as this'll give a lot less problems later on.

It might take some time to install it, but this is only a one-time installation.

Using standard connectors completely makes it easier to ugrade/expand/repair your network later on.

There are some nice connector shields out there that have coax+ethernet connections, enabling you to simply plug in a standard cable anywhere in the house.

also, if you ever want to sell the house, it is worth more, as it has the cables already ;)

In addition to what others said: TV coax is not the same as 10Base2 coax. The former has an impedance of 75 Ohms (at least in Europe); the latter has 50 Ohms.