Just to chime in here, I've used a Windows phone (Nokia 920) the built in navigation app is great and it allows you to download maps for off-line use. I used it in Italy last summer and it worked perfectly. With the new larger phablets UI elements shouldn't be a concern, they aren't for me at least even on the smaller 920. I think the HERE navigation app is available to all Windows phones now, it was locked to Nokia only, not sure on what they might charge though.

But sounds like a separate stand alone device fits your bill the best!