I totally misread your post, I have iPad Mini on the brain. Sorry! As for my iPad 3 , I went wifi only when I bought it launch day and there have been several times I regretted it. So yes, I wish I would have gone cellular.
IMO i don't think so.. I found a few youtube videos that a guy was using a external GPS module on his ipad and was getting turn by turn navigation on a Wifi based tablet..