I'm in the same boat.
The OEM parts seem to be $345 for the front half including cat and $310 for the rear half, or somethign like that.
I believe thats the BMW price I found somewhere.
Check out this site.
http://www.autopartsdeal.com/
Consider this.
If you're restoring, buy the OEM and mount it yourself if you can.
You should be able to remove and bolt on as long as the mounting kits aren't welded to shit.
And you may need new mounting gear ( Brackets and so forth, they seem to rust with the exhaust)
If your not worried about a numbers matching part then I think the E32 parts will work.
Look at the Flowmaster Cat and you'll need to have everything welded in. Then you can pretty much choose what you want.
Good Luck
Howie.