It's really 50/50 whether you need a chain tensioner or not all depending on the chain ring/sprocket combo. The RD is the one that originally came with the bike so I left it on instead of spending money for a chain tensioner. You also need to order some special non-turn washers from Shimano.

Since I took off the rear cassette, FD, 2 chainrings and front and rear shifters, the new set up is actually a little bit lighter than the original but most of the weight now is in the back. I don't care at all about the weight since I live in a pretty flat area and heavier you bike is the longer it coasts and requires less energy to keep it at speed though it does take a little more enery to bring it up to speed.

This rear carrier is the best thing I ever bought! It is so sturdy and actually specifically made to attach the way you see. It securely fastens down on both sides and can also be locked. I have loaded it full with 4, 2 liter soda bottles and still had room for other things such as chips and salsa and beer.

I used to use a fabric trunk bag from Performance and the thing was awful. After like a week it lost its' shape and started sagging to one side whenever loaded and the velcro fasteners stretched out in no time.

Here are some pictures for you: