I think if you go with steel bases they are always better than aluminum rings, but that being said, plenty of people have used aluminum bases without a problem myself included. I don't like aluminum rings though, and I'd go with a quality set of rings. You may be able to use lows with a 40mm scope, but I'm not positive on that.

To me lows, put the scope to low for me, but some guys like that and think mediums are too high. It's all a personal preference.

For rings, I'd highly recommend the Burris Signature Zee rings with the plastic inserts, they are wonderful. They eliminate the need for lapping, are strong, and best of all don't leave any ring marks. It's the way I'd go, and I've got several sets of them. I won't buy anything else at this point as they are the best I've used under $100. The only downside I can think of is that the lowest they make is mediums, but if you can use those, these are great rings.