Cat Replacement Pipe: Go to a local exhaust fabricator specialist beacuse in most cases they will be able to knock you up one cheaper than you can buy it off the shelf for.
Exhaust Back Box: Assuming you mean somthing better than the standard OEM unit then have a look at Ragazzon (just search Google). Quite a few 145/146 owners have them fitted and they get the seal of approval. You can go with a number of different configurations from all 3 silencers to just 1 depending on your requirements and the amount of noise you're prepared to put up with. Alternatively go to an exhaust fabricator specialist and get them to custom make you something to your sepcification (some of these aren't as expensive as you might expect).
Aftermarket Air Filter: I recommend keeping the standard airbox (i.e. don't bother with a cone or expensive induction kit) and that you just fit a free-flowing panel filter in place of the current paper filter. I have a K&N one in mine although BMC is another good make. I've got the K&N one in mine and it cost ~£40. Have a look on eBay for cheapest prices.