So motor wise you're in for
Used S63TU - R200K (they are all advertised for more locally)
Build on the S63TU - R100-150K
Turbo upgrade - R200K (Weistec setup)
MOTEC - R90K
This is before you have fitted anything you're already past R600K
That is stuff you know the price of
Then there is:
Gearbox: a Hewland sequential box is around 25K GBP which is about R585K at current rates. It goes up from there...clutch, custom fabrication to get it to fit etc. No off the shelf gearbox from BMW is going to work (I don't think)... even then a DCT Box might be 100-150K and then you spend another 100K on clutch and basket upgrades and then another amount of money for a tunable controller of some kind. Even if you got one for half price, that is a chunk of change
The custom work on the aforementioned Subframes, drive shafts etc - that Alfa build is an example.
ALL the custom engineering work just getting it to fit - do you have someone who you would be able to get to take on work like this? Or can you perhaps do it yourself?
ALL the custom body work for the front and the back - I honestly wouldn't even know where to start in terms of getting this done in SA?
Suspension + wheels & tyres - gets VERY expensive for your kind of use case
Cooling?
Oil system?
Brakes (R40-50K front only is a starting point for things like AP Racing)
Tuning for engine and gearbox - no idea
Then the cost of the i8 itself - I dunno what that is, but even at R500K, I would be amazed if it still made sense. Then regarding balance, why do you feel this will be better than either the McLaren I posted or the Porsche GT3 RS (or hell GT2 RS if you have that desire for power) and spending time tuning it for your driving style and prepping for events.
I don't think using a V10 is going to be any different (if anything it would be more money). Using a 6 presents the same challenges especially getting it past 1000hp
I don't know what you could reclaim from sale of i8 parts.