Hello
I have many, many questions lol, please help me^^

I have a Ford Focus MK1 1.8L 2000 Model and it had it's MoT in January. I was told that the exhaust was blowing.
Today I checked it out because the rattling was annoying me now and I found that my Central and Rear boxs wear blowing and leaking. I found this out by covering up the back box exit with my hand to find that water sprayed out from both boxes. They are both extremely rusty too.

Now I'de really like to attempt to repair this myself as it doesn't look too hard according to this link:
http://myfordfocus.com/how-to/cat-back.htm (Assuming it is correct?)

But what is confusing was it is telling my to saw off the Back Box from the Central Box before the rear suspension.
Is this correct?

Then this led me to wondering how would I connect these parts together on the new set?
Do I need some sort of glue or clamp?

It is also meantioning that I'll need a new Gasket, but was wondering if I could re-use the old one?

Here are the parts I'm looking at:
Back Box: http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=160468385739&category= 21654
Central Box: http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=150480021996&category= 21654
Neither meantion anything about connecting each part together or in need of a new gasket.

If I went to a garage, roughly how much would this all cost to get it done?
If it's cheaper to DIY, then I'll do it.

I really need these questions answered. I've got some experience on repairing cars but this is my first exhaust.
Thanks