LSx swap is doable, but it's logically necessary to swap a suitable transmission incl. transfercase too. So there's a need to manufacture the custom propshafts and to use different front differential or, which is more difficult (even if it doesn't seen so) - to make proper diff mounting for the stock front diff. The other problem seem to be a steering, but since the stock steering isn't that responsive, it becomes less relative.

In terms of electronics, a true programmer can make any engine run on any ECU, keeping all the functions, if a physical wiring is done.

The most concerns on this way is the emission standard with at least LS1, so it's not legal in Europe, at least on a facelifted E53 with Euro 4 emission standard. Since there's also some strict emission regualtions in some US States, LSx swap wouldn't work there either.

The other thing (USA unrelated) - even some BMW engines are unreliable on their way, but I personally would never swap an US eingine in a car which is supposed to be driven on autobahn, because I had an experience with MP112 supercharged LS2 powered GTO in Germany and in my opinion the LS2 engine is a complete rubbish - in stock form it had overheating issues after several minutes of constant full-throttle driving on autobahn. So the LS1 wasn't "vollgasfest" either - that was the reason, why it was never installed on Opel Omega in Europe. The torque feels impressive, but after 200 kph (125 mph) the 470 hp supercharged GTO is much much slower, even than heavier stock M5 E39 with like 370-380 real hp.

Regarding the M5 E39 I can even say the S62 engine may seem rubbish in terms of reliablity too, but with right knowledge and appropriate care it's very durable - I also have a M5 E39 with 384 whp and it runs great with almost no oil consumtion and it is very capable of so called "Dauervollgas" ("long-term full throttle"), which at least LS1 and LS2 aren't capable at all.

I would rather consider a M73 5.4 litre V12 engine swap in X5 (only with Euro 3 standard), these engines are even cheaper, than LS1 and are very tunable and very reliable - I've already checked this option and with 2000 Euro investment you can easily get 400+ hp just from agressive cams and head work.

M73s have very poor head flow and basic SOHC heads, but they're hardly heavier than M62s.

In case of X5 - there's no need to use different colling system, but there will be some difficulties with front diff mounting and steering too.

The bad thing - V12s sound shitty compared to V8, but there's finished ITB systems for them and with some more invsetment you can get a carbon fibre intake airbox for ITB, with higher compression rate and lighter pistons 460-470 n/a HP are achievable for about 7,5-8k Euros even with stock SOHC heads. And this set up will sound exactly like the S70/2 powered X5 Lemans....

If I may continue my Christmas phantasy, I'd also check how much the custom made S50 based DOHC heads would cost for the M73 engine, so it would be a total X5 LeMans convesion.

My feeling is that a complete X5 LeMans 6,1 L DOHC V12 AWD conversion would cost about k28-30 Euros incl. manual tranny conversion but without workshop labour, even the power would be rather like 600 hp, than stated 700 on the original X5 lemans.

X5 with 5,4 L 510-520 HP n/a v12 with ITB, DOHC and carbon fibre airbox with manual tranny would be 6-7k less or rather ~25k.

X5 with 5,4 L 460-470 HP n/a V12 with ITB, SOHC and carbon fibre airbox with manual tranny would be 15-17k and, as I already said, this set up will sound exactly like the original X5 LeMans.

X5 with 5.0 L 500 HP S85 enigne and manual tranny would cost also about 15-17k.