the box is not the end of the world.. do a search for threads on it. there are loads of step by step guides and I put my own guide on here.. From start to finish, allow yourself a good day to change it if you have not done it before and make sure you have a good supply of fluid.. Dont forget once you remove your pipes from the top of the box, fit some new O rings. This will prevent the top pipes leaking too.
Also, one other tip for you... If you can, buy a box with a drop arm attached. It may cost an extra tenner, but will save you hours of swearing to get the old one off!!! You can get them off.. I did, but you need a heat torch, big hammed and a gear puller! They are not easy!
Good luck..
MudRat