Sorry for the very late reply, the thread didnt appear to get much interest so havent looked here in a while! I will try to answer as many questions as I can (I'm still learning too!)
To reply to the info regarding the conversion:
Well essentially the hardest part of the whole conversion is getting a complete ECU kit, i.e. the DME (or ecu) the MATCHING EWS unit (the ews and DME are coded together, either is useless if the other is missing!) and finally the MATCHING key for the EWS! dont listen to any rubbish about the key being paired with the DME its NOT! I have alot of information on the immobiliser for this car and I must say its a very clever system!
Mechanically as you know the engine will fit into the shell, there are unused studs and mount points some of which you keep others you now use.... (i had to look at alot of pictures!) You will need the 6cyl gearbox and the propshaft and gear linkage, i chose to keep the 318 diff and half shafts..... i was lazy lol! i have the 325 rear end parts in the garage still!
Some attention has to be paid to the cooling system, you will need two new radiator hoses (top and bottom) a new header tank (on the 16v 2.0 i think the header tank is the same as the 6 cyl) and another hose that is the return feed from the heater matrix, also you will need the feed and return pipes from the steering rack and the return pipe from the pas cooling ring in front of the radiator.
Air con is pretty simple, just one pipe is required that is a shorter version as the 6 cyl block is longer. and also a 6cyl anti roll bar or sway bar. I think thats about it.
All the engine bay electrics are different, they are simple factory harnesses that are easily sourced. The drive by wire need to be installed! thats 6 wires! and a few other wires for the cruise control as the DME controls the cruise and not the separate unit as before.
I didnt do the 3.0 litre as I couldnt find one for a decent price! the 2.5 set me back £500 and was a bit of a bargain!
With regards to approving changes.... it really does depend, the V5 registration certificate should be updated with the new engine details and capacity, this WILL change the road tax duty band! also the insurance company may demand the car is inspected (called an engineers inspection) this will check the quality of work done to the car, only then will the insurance company be happy to cover you, it really does vary with insurer though!
Suspension is still factory fitted AC schnitzer springs and shocks, its lowered about 30mm i think? Brakes are standard 318 front and back however have everything to convert to 325. any thing else you want to know give me a shout!