Polariser and UV 0

I am going to buy a Nikon D70 wit a kit lens AF-s DX 18-70mm and a Sigma 70-300 APO Macro Super II. I love nature photography and this way it is a must to have a good polarisation filter. I tought to buy myself a Hoya circular polariser. Can I use the same filter with both lenses with a step down ring or it is better to buy two filters?

The second question is about a UV 0 filter, which I would use it to protect the lens. Which one is good?

And at least: are these lenses good for this money, or are there any better lenses for same money?

Best regards

Gregor

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Live your photographs.