If you are doing an engine swap you would need to rewire it to make it work, upgrade the standard cars transmission (It's not necessary, but I would), maybe even doing custom work to make the slightly bigger engine fit and mount properly, upgrade and set the suspension to the different weight, and if I needed to do all that and basically make everything from scratch I would just get a proper BMW V8 like a N/S63 since you did say you wanted the fastest X3 and that money wasn't an issue. Get that baby to 500hp and you're set
So if you want an answer if it is possible, well everything is possible but I don't think it is going to be worth it. You would need an excellent mechanic that really knows what he is doing for this to be doable, but for the money and the trouble with it you could get a 5 or X5 series with the same or even a stronger engine.
I may be wrong so feel free to correct me
