Hi @Kevin, here are a few answers - I'm sure that others will come up with other ideas:

Are there 100 % natural girls here? who are they?

If, by natural, you mean no silicone or tatoos, you have hundreds to choose from. Check out the "Categories" in the Girls Store. You can check whether a girl is listed when you select "tatoos", although I don't think you can limit the selection to "no tats". Similarly for "big boobs", many of which involve implants, although quite a few of them have naturally big ones (like Lucy Li), so it's less reliable.

Who has beautiful and sexy feet?

Again, use the Categories tab to check for bare feet in any act. The only way to see if a performer's feet suit your taste is to download the preview.

How many cards include blowjobs? who does it better? Who fucks herself?
Who uses more than two dildos and does a double penetration?

Use the "Collections" tab to select xxx cards only. Many of these involve toys of one sort or another, which get inserted at one end or both ends. Again, not 100% reliable.

Which is the best duo?

That's down to personal taste. If you want a recent xxx duo, try the Lucy Li / Izzy duo, rather than the Kristina/Dianna duos which are strictly striptease.

Users' tastes vary hugely and there is something for everyone, but it's really hard to suggest which cards are "best" for you. The best bet is to select girls you like, maybe restricting your selection to girls with xxx cards, and then download a few previews. I hope this helps.