Android, like all mobile operating systems, has to be built for the hardware, i.e. for the specific tablet. There is no generic build that can run on any tablet, so if you just buy a Windows tablet and assume you'll be able to run Android on it natively you will almost certainly be disappointed. Of course you may be able to run Android in an emulator, but that's not what you asked for.
There are a few dual-boot tablets out there, so it's possibly simplest to start by seeing whether any of those meet your requirements.