You don't need waterproof, you only need weatherproof. Commercial garage door keypads aren't even waterproof, typically just have a flip cover and plastic that keeps rain pathed out.
That being said, any keypad can be made weatherproof with the proper use of seals for the enclosure.
Pick a keypad you like, then work on the enclosure.
Yes and no, probably.
Looks like it has a USB interface, meaning it's probably treated as an HID keyboard, meaning yes it can connect to an Arduino, but only the Arduinos with Atmega32U4 chips like the Leonard or ProMicro.
That's quite an expensive unit, is that within your budget for a keypad?
You can have it delivered with a ps2 connector aswel. I just got one of my old keyboards and cut of the connector. Found out you can connect the wires from that connector with the arduino and use it. So it should work with that one aswel i guess.
I reckon with a good fat bead of Silicone bathroom sealant all around the edge of the first (very cheap) one you showed you'd be fine. For a couple of quid/euros/dollars it's worth a try. fit it up to a waterproof project box, do the sealant thing and douse it with a hosepipe for an hour or so. If it still looks good and works, you should be good to go.
It's easy to wire up (as long as you have a good debouncing routine) and will save you a truckload of $$$. If it doesn't a) you wasted the price of a coffee b) you learnt something new.
If you really need a water tight one, you could build on that works on capacitive touch. They sound and look hard. Once you have done a few they are surprisingly simple to get set up and can look like just about anything and will interface well with arduino. The only thing they lack is tactile response.