Ugghhh.. how much would it cost to fix this?

Hard telling. For example why is the cat bad? Most cats go bad either because of overheating or contamination. If it was contamination, this is usually caused by burning oil which in essence will destroy your new cats almost immediately if the original problem is not corrected. It may also lead to a complete rebuild. As you can see there are tons of other variables as to why everything else crapped out. Also I don't know what your flat rate is there or canadian conversions for that matter.

So stock replacement stuff to get the car on the road for a start?

Brakes are easy.. $90 for both rotors, $40 for front pads, $60 for all new wheel bearings. Front brakes are very easy, something you should tackle yourself.

Springs and steering stuff you could get from a scrap yard. Having mustang buddies helps when you can't do the work yourself, otherwise shops will charge a fair bit.

Stock replacement shocks are pretty cheap, but the struts can get expensive. You'd want a shop to do these if your not mechanically inclined.

Convertors.. new h pipe with cats is under $250 from dynomax i believe.

$1000