If I buy a Harris basic bipod not the S-series if at some point I want to convert it to s-series is that a possibility? And where is the best place to buy a used one?
You will not be able to make the conversion. The best you could do is sell of the standard when you are ready to upgrade. In my opinion it is well worth it to just bite the bullet, and get the S version from the beginning.
Just my two cents currently I am using a Champion bi pod. They swivel both directions. I believe 30% both directions from center and adjust left and right on the cant also. Harris only swivels on the cant. Also comes with the lever in the front to set the tension on the swivel. I know that i will get laughed at because these are not a Harris or better but I am on a budget and it works real good for me. It works good when shooting P Dogs you can scan the town for potential "victims"