Apart from that, you just need to ensure you get the right de-cat pipe for your car - the cat (and hence the decat pipe), come after the lambda sensor, so don't worry about it.
Once you've got it, fit it yourself, or go down to your local exhaust place, who'll fit it for £10 or thereabouts. Keep the cat. in case you need it for MOT - shouldn't do though as our 1993 Astra Gsi got through recently with de-cat.
And don't let the place who does the work for you keep it!