1. No, it won't short out your phone, nor will it crash your computer. What it will do is stop the transmission of information between computers and the internet, during the time that the cordless phone is in use. I.e. if you are on the internet and a phone call comes in, the internet connection will stop working until the phone call ends.
2. Yes, microwaves will cause the same problem, when they are working. So if you start popping popcorn in the microwave you will generate the same symptoms as above.
Pace makers too, I've heard rumors that some say florescent light causes some problems, but I can't vouce for that.
3. You have a choice, either you could go with the 900 MHz phone system, or instead of buying wireless 802.11b or 802.11g (which operate at 2.4 GHz) you could buy wireless 802.11a (which operates at 5 GHz). It cost more because not only does it transmitt at a higher frequency, but also greater bandwidth - about 54 Mbs - 72 Mbs range, as opposed to 11 Mbs.
4. All wireless is pretty much the same. Some have more capabilities than others, but as far as performance, no one is really leading the pack or falling behind. The only thing you want to be sure of is they are WiFi compliant. That means you can use any brand to work with any other brand. Pretty much everyone is though these days.
If you are interested in a wireless access point with router capability, I can get you a NetGear 802.11a for $276.03.
Regards,
D