Ok, first, you just need a basic Harris bipod. You don't need an adapter or anything else.
Second, for varmint hunting, I recommend the hinged or swivel model. Basically what it does is it lets the rifle adjust for cant. If you are on an unlevel surface, you can adjust the gun to keep the crosshairs level. It does not rotate left to right, but the levelness of the rifle. Any Harris I have is a swivel model.
Third, Length. The length is up to you. If you are going to be hunting where you want to make shots while sitting on your butt, get the model 25(s). It adjusts tall enough to do that. If you are just going to be shooting prone or off the bench, get the model L(s). Unless you are only going to be shooting benchrest, I wouldn't recommend the benchrest model.
$108 for a swivel model is too expensive. Shop around some more and see where else you can find them. I wouldn't pay over $90 for one. They are worth the money, but you should be able to find a swivel model for less than $90.
Good luck.
This is the Model 25(s). Note it is pretty tall without extending it. You can extend it tall enough to shoot sitting down.
This is the Model L(s). Note it is not as tall. I prefer this model for anything other than hunting coyotes where I might want to sit in the shade of a juniper on a tall hill. This is the "prone bipod" if you ask me.
Old threads.
http://www.thehighroad.org/showthread.php?t=54731&highlight=harris+bipod+model