Just a few questions, first off would there be any reason this wouldn't work? Or is there any other option i should consider instead? Also i dont think there is anyway to run it through the walls without significant cost.
cyril7: As reg pointed out 100m is the maximum length that is rated for Ethernet cabling, however this is a conservative figure that the standards require you stick to (for professional installations) to ensure that it will work as intended, however you can go past this distance with varing results depending on the performace of the NICs at each end, and the exact performace of the cable and installation methods used.
Stranded cat5e cable has higher loss than solid, which is why its only used for patch cords and to connect between the TO socket and your PC etc, obviously being more flexible its better suited to that role, albeit with a higher loss. According to the standards stranded cat5e has 1.5x the loss of solid, so if you were to use an entire run of stranded to comply with the standards it has a maximum length of 67m.
End result is 30m of stranded patch cord is fine.
As for interference issues, in general UTP cabling is pretty immune, just pays to keep the noise down in the primayr spectral areas, ie 31MHz for fastethernet and 62 for GigE. Try avoid laying near power cables, and definetly keep away from switch mode supplies in particular those used in 12Vhalogen lighting applications and also fluro tube fittings, give all these a goo 300mm clearance, otherwise the standards require 50mm clearance from power cables for safety reasons.
Use a tool like wsttcp to test the speed between to PC's it bypasses the normal stack and can check TCP throughput and show if you have any dropped packets, this is an easy way to see if you have problems, otherwise a Fluke DS will set you back $20k or so.