There is one flaw in your idea.
The PC will require power in order to provide any of the security funtionality you're aiming for. When the car is runinng, that's not a big deal. However, that's also not when you want/need the scurity.
It's when the car is parked that you need the security functionality. If you have the PC running when the car is off, you're going to be draining the battery, unless you have a source of AC power, meaning an extension cord running to your house or apartment.
Now, I see you mentioned WAKE functionality. The idea is sound fo hte WoL, however, there are a few flaws:
1 - In order for the camera to attempt to FTP an image (or save it to a network location), that location has to exist already. If the PC is in hibernate or standby mode, then the location isn't there, so no image will be saved.
I don't know if these sorts of cameras have any internal buffer to store images until they can be saved to a network location. If so, then this might alleviate that problem, but then why would you need to initiate a WoL in the first place?
2 - If the camera can't buffer images, then the first couple attempts to transmit via FTP may result in the image being lost, as the "save to" location isnt' ready yet. That could result in precious data being lost.
3 - If the car sits for a period of time, the cameras are going to capture images of other things such as animals, other cars parking, pedestrians, etc. Each time the motion sensor caputres an image, it's going to boot the PC, save the image, and repeat whenever it senses something. That's going to drain your car battery. Shoot, keeping hte PC in standby mode will drain your battery, as well.
How quickly? I can't say. But if the battery drains to the point where the PC can't boot, then the system is pretty useless.
If a criminal knew what comprises this security system and how it functions, it would be pretty easy to override it by creating motion to keep the PC running, drain the battery, jump start your car and go.