problems with converting a N/A car to forced induction (will use turbo as an example, but S/C is easier coz u dont need exhaust manifold)
1) fitting everything into the engine bay (turbo, I/C, hoses, etc.)
2) youll need a custom exhaust manifold to mount the turbo on ($$$)
3) youll need 3rd party management (what you refer to as another ECU) and someone to map it to your car ($$$$$$$$$$$)
4) everything else ie brakes, suspension, gearbox, clutch will need uprating aswell
5) compression ratio of engine will either need to be lowered (new pistons) or you will have to run REALLY low boost
6) you will probably need injectors with a higher flow rate if you intend to run more than a few PSI of boost
7) its bleedin expensive!
i converted my XR4 to turbo and that was with a 2nd hand KIT designed for the car itself. its very satisfying, but it does cost, especially if it your 1st time at proper engine mods (fitting a backbox doesnt count as an engine mod
)