Robbie Boy

Good evening - last October some nice people relieved me of my catalytic converter and SS backbox on my 04 MGTF. I've just bought a new aftermarket CAT but wanted to know which oxygen sensor i need to buy - I know they have cut the wires but as i put the car away over the winter have yet to fully investigate the damage underneath .
Also i'm looking at a Gravity SS back box on e-bay - does anyone have any experience of them - are they any good /too noisy etc  
The Cat comes with a fitting kit but the exhaust does not - what else might i need ?