Verizon is easiest, but you can mod sprint, alltel, quest, telus, etc.. basically anything except for T-mobile or Cingular/At&T.
The phone should come with a USB cable.. it's required to sync your phone to the computer, something you'll probably do nearly every day... No other hardware is needed. You might have to download some software if you get a locked phone, which is the reason verizon's phones are easiest. THey never lock their phones.
In short, I'd get a verizon phone unless you get a SCREAMING deal on another carrier's phone.
Pretty much all the info you'll need is on this forum.
One disclaimer, which I've realized in the last week is probably pretty important:
DO NOT ATTEMPT TO UPGRADE THE AKU UNTIL YOU KNOW WHAT YOU'RE DOING. Several "noobs" (no disrespect intended) have had problems, which would have been avoided if they had waited, gotten more involved in the community, and done a little research on other forums before they tried to take matters into their own hanRAB.