I have one of the HS Sentinels myself. They were made for HS by Dan Wesson. As such, it will take Dan Wesson barrels, but you will have to research a bit to see which Dan Wesson barrels will fit. Many of the HS Sentinels used transitional barrels; that is, a porkchop barrel with internal lock nut. However, it is also possible that you have an earlier model that uses the porkchop barrel and external lock nut. Like a said, you might want to bone up on them a bit.
For a fixed sight gun, $250 is close to fair market value. Not a steal, but not overpriced either. If you like the way it looks then buy it. They are well made guns and you can't touch a S&W or Ruger .357 Mag in similar condition for that price.
-Lee