Pre and post facelift bumpers are interchangeable yes, but then make sure you obtain the correct grill as they’re not interchangeable. Also note fog lights and even the bonnet are not the same (subtle difference in the bonnet lines and the front leading edge as the pre facelift has a chrome trim).
When you say it keeps spinning, does it sound like the engine has lost compression? When it cut was there any smoke? Or sound like a turbo popping? Has it lost coolant?
Try temporarily blanking the egr valve at the front with a small metal plate and see if it starts. Then you’ll know if that’s...
The AC is only required for a forced regen, not for normal regeneration during a regular driving cycle. I’m just speaking from years of experience with these cars, once you’ve needed just one forced regen, the time is up for that dpf.
If you can find a cheap engine and reasonable mechanic I would budget minimum £1000 all in. Too many variables to give you an accurate figure. I would suggest shopping around and getting a proper diagnosis first.
Probably a blown turbo or possibly oil pump chain. Is your oil pressure warning light on? Without stating the obvious you really shouldn’t be starting the engine if you want to try and salvage it in some way..