I'm confused on what you're going to do... go from the 1.8 to a 2.5? First off, don't get a 92 as it's the non-vanos engine and lacks in pure power compared to a functioning vanos m50 engine. Second, you're going to need to have custom engine mounts fabricated which isn't a big deal for someone who knows what they're doing. Third you're going to need EVERYTHING for the engine right down to the wiring harness and MAF. Fourth you're going to probably want a new rear end as well as transmission, brakes and suspension because this engine obvioulsy has MORE power and is 200-300lbs heavier then the 1.8.
Good idea? I would say NO because all of this and labor might cost as much as a decent used 6cylinder BMW.
Thoughts? Get rid of those god damn one piece headlights, turn your car back into stock, sell it and buy a 6cylinder. I see you're from New Zealand, so I'm not sure about the econmic's over there, but what you want to do would probably be cheaper and BETTER if you just bought a whole new car.