I have a Nexus 7 (not the brand new version), which I love. It is the version with a slot for a mobile phone card, but of course you don't have to have it connected to the network permanently. Mine is on wifi 95% of the time. I find that a basic GiffGaff data package at only £5 a month is plenty.
The new Nexus has 4G, I see.
The Nexus uses Android Beam rather than generic Bluetooth, which I assume means that if you want to tether it to a phone, you'd need an Android phone. I'm thinking of getting one of the so-called MiFi devices, which allow sharing of one mobile signal (and therefore one mobile contract) over several devices, but using wifi rather then Bluetooth. But I've no practical experience of either of these yet, I'm afraid.