For the extension, I would strongly recommend the Nordic Components +2:
http://buy.nordiccomp.com/product.aspx?pid=343
For the carrier, in order to avoid the potential hassle of dealing with stripped screws, I might suggest (particularly for the Benelli SteadyGrip stock) going with the Mesa Tactical buttstock mounted 4-shell carrier:
http://www.tacticalshotgunner.com/me...uns-p-209.html
I have tried all of these on my Benelli SNT (identical to the one you showed) and I stand by NC for their extensions and MT for their carriers. Now, if you decide against the buttstock mounted carrier and opt for the receiver mounted version (I have tried both on my Benelli), I would go no further than the 4-shell carrier as you may run into problems in terms of balance and/or forend battery issues. Also, I would strongly recommend using LocTite Blue on those mounting screws upon installation.
For the tritium sights, you might try Meprolight. I use these on my Mossberg but have yet to worry about them for my Benelli.