You seem like the kind of person who will enjoy the N810. But you might want to wait a few months for a new device...
If price isn't a huge issue (think ~$500 instead of ~$220 for the N810), I'd recommend waiting for the new tablets as both the N810 and the iPod Touch are really out of date at this point
Turn by turn directions are supported by navit (but you may not find good maps for navit)
I would however consider getting an N800 instead of N810. You can use the money saved to get a keyboard and bluetooth GPS (which you'll probably need anyway) and you can expand the storage space to 32GB (2x SD cards)
you can pick up an N800 or N810 cheaply now on eBay and, if you want to jump on the new device when it comes out, you'll probably be able to re-sell the old one without suffering too steep a loss.