It may be counter to other advice given here, but I say rent their modem.
Chances are you will get an upgraded one from them at some point before you would make up the purchase cost of the original and it becomes their problem to make it work, forever. May also make support calls less stressful as it removes another item that is your problem.
Annie - With Comcast I bought the Belkin router and it has been absolutely flawless. I must add that the cable lines in our development are almost new - about two years old - and buried.