You won't be able to use BNC (even if you could find NICs that work with modern OS's), as the impedence used by BNC co-ax is different to TV aerial co-ax. The terminators won't work and the cards won't see a connection.
I've never been impressed by homeplugs. Perhaps it depends on your level of networking knowledge, but when it's your day-job and livelihood, homeplugs are cheap and dirty.
This unfortunatly, one of our guys was in a similar situation and we fitted home plugs about 12 months ago. 3 sets of plugs later (2 have failed completly.... Burned out, no life at all) and I still get regular calls. Now it could the wiring or some other reason but we have just found them hugely unreliable.
Personally I would go with some enterprise grade wireless N gear and see how you get on.