If the computer cycles to zero after 1000 mi/km, I wouldn't think that's a deal breaker. Just keep track of how many thousands of miles you've gone in your head. Even if the display supports distances over 1000 mi/km, it's a bit risky to depend on the computer to keep track of that on its own. If you have a digital camera, just take a picture of the display when you think of it. That will give you a log of distance at a specific date and time.

It's hard to find a computer anymore that isn't at least water resistant (that is, sealed against water getting into the unit itself). The main water problem is when water gets in the contacts between the computer and the mount and messes with the electrical connection. I have yet to find a computer that is really rock solid against this in a pouring rain. Not much to do about it apart from tie a baggie over the computer when it starts getting wet.

The latest computer I got is the Sigma Sport BC906, which can mount on the handlebars or the stem. Mounting on the stem frees up space on the handlebars for lights or, um, for your hands. I'm also partial to computers with nice thick wires that don't pull so easily.