The reason the BMW cats cost so much is that they are metal cats.
the cheap ones are ceramic and wont take the abuse the rpm's
of our motors would give them for as long as you would like
Yes any good exhaust shop (Midas is not one....) can weld in a new cat.
A good metal cat will be between 175 and 300 depending on brand.
From the cat back there is nothing magical about the BMW OEM
exhaust.
Example: I just put a complete new exhaust on my Jeep. It cost just
shy of 700 bucks to have it done, This was from the header flange
to the back, FlowTech Cat / metal , Upsize to 2.25 in pipe, Spintech Muffler.
Labor ate me on this a bit because a Jeep needs a LOT of bends
and I had a couple special requests that made it harder.
The 318 should be cake, its only got a couple bends and a easy path
to fit it into. Spintech sells oval pipe if yyou really want it tucked up tight...
Dave