Welcome to the forum SonicMarmalade.
1. As no doubt others will point out, there has only ever been Vauxhall Omega B. The term Omega C is sometimes used to describe the post-2000 'facelift', and on these forums we mainly refer to is as 'facelift' rather than Omega C.
2. In my opinion it should be possible to swap parts between pre-facelift and facelift models - but there will be a lot of them, e.g. bumper with washer bottle, headlights, fog lights, many more - this means lots of bits and lots of work - and to be honest I can't see that it would be worth it. The result would be a very odd car to the eye...
3. Going the other way round, i.e. fitting 3.2 engine to a pre-facelift car, is not straightforward either - different engine management means that the ECU, loom and sensors have to go as well, including the exhaust with the two lambda sensors... again not simple to do, and while not impossible probably not worth it.
Incidentally, the 3.2 engine isn’t dramatically better than the 3.0, it is indeed a little higher on BHP and torque but in fact it has lower compression ratio than the 3.0 (10.0 as opposed to 10.
, and only makes up for this by tighter management with more electronics and improved ignition/injection etc – all done to reduce emissions.
Also, the 3.0 can be chipped – see M-tek Performance in this forum - while the 3.2 can’t (the chip on the 3.2 is flashable and integrated into the ECU, so can’t be simply swapped as with the older ECUs – any remapping need to be done on the car using Tech2, so not impossible but more complicated to do and due to lack of demand not currently available)
I suggest that you simply find a ready-made Omega to suit your taste - they are in ample supply and don't cost very much.... a nice MV6 ex-plod 3.2 manual on Y or 02 plates, or an Elite 3.0 Auto on S/T plates (i.e. 'mini-facelift')... and if you want more power than talk to M-tek.