Grey area I'm afraid. Any ariel that could pick up a very good analogue signal, and is in a freeview area, should in theory be able to give you a good freeview image, but in my experience it's rarely that simple.
You could be lucky and get great Freeview first hit, or you could be unlucky and get a garbled mess of channels, and none of some. Basically though, as far as I understand it, if you get a good analogue signal through the ariel you intend to use, and you're in a strong area, you should be fine.
As fas as the box goes. Don't buy cheap crap, as you will get unreliable results from crap tuners. Again, just in my experience and of those I know