I don't really see how much you would be saving... particularly if you find you have bought components that are not compatible or you have unworkable software... that would be a complete deal-breaker right there. Even if you fill and inside straight and pick components that get along, and find software that works... unless you have some wild hardware requirement... I don't see how you can beat the value of a prebuilt system.
The other thing to consider... is if and when you actually NEED that video, and find because of incompatible hardware or software... your video is bad or non-existent because something went bust when you weren't looking.
The Swann system I have has PC connectivity, you don't even need to touch the VR unit to view files. I don't remember if it has live monitoring (on a Windows PC) because I don't have mine set up that way, but I believe it does.