For a facelift I reckon you will need -
The bumper - £50 to £100 for one that needs attention ?
Lower grill - don't come up often, but I snagged on for £40
Side clips - worth replacing even if not broken, the facelift ones are more durable - £50 from the stealer
Arch liner push rivets - 10 for about £3 from the stealer
Fog lights - don't come up often, think they are ££££ from the stealer
You may need side reflectors - does the pre-facelift have these ? I got some off eBay for £15
Oh, and on the bumper - headlight washers or not ? If yes, you need the covers but should have them, if not then you need a bumper without holes, or the covers glued in place.
I don't know if you would need a new undertray or corner 'air ducts' - guessing the pre and post facelift are the same. If you did need a new undertray plus a single air duct, they cost me about £120 all together from the stealer.
For the ///M bumper you definitely need a new undertray as the facelift is too short, and you need the two side air ducts. The bumpers are also way more expensive - IIRC I have seen them from £250 second hand, up to £600 for a brand new one. On the plus side, no need for fog lights
I am sure someone will tell me if I got any of that wrong... However one way of checking bits and part numbers is to go to http://www.realoem.com and choose the model / year you want - lots of exploded parts diagrams for the whole car.