For HD as you trave, you could go with a Winegard Trav'ler rooftop automatic antenna - it works when stationary. Or, for less money and more hassle, you can get a DirecTV HD antenna and a sturdy tripod and just set it up when you need it. Lots of folks do one or the other.
As far as the HR-24, it will work just fine with the newer antennas, and if Winegard says it will work with your old one, you'd be good to go. The HR-24 is not DirecTV's latest recever, but lots of folks have them. Not sure where you'd get one new. though. And if you get one of the newer "Genie" DirectV receivers for your sticks-n-bricks, DirecTV won't also authorize the HR-24. But if you get the HR-24 authorized first and then add the Genie later, they won't turn off the HR-24 (at least they didn't for us).
al