I don't think you can just swap the "brain". Yes the "head unit" is just a display and input device, but suspect that the drivers might be different - basically the "brain" is a small computer running a flavour of Unix.
Like any Unix system, it will need specific drivers for screens and input devices. These are unlikely to be found on the "brain", which will be loaded with what you need for its normal pairing.
And that's before you start dealing with any VW attempts to prevent theft/upgrades...
I'd definitely say that trying this using components you buy from Ebay is unlikely to work and will cost you a fortune.
The other thread suggests companies that might have the ability to do an OEM upgrade, I'd punt for that if you really wanted to do it, apart from anything else, you can get your money back if they screw it up!