This sounds weird, you have a car w/ heads for twin turbos?? The system consists of the 2 turbos, 2 waste gates, 2 down pipes, a bov (optional) and a set of headers or log manifolds for the turbos, a single dual inlet or 2 separate intercoolers are also optional. Then there's the intake plumbing and the oil supply & drains. This means that your engine either sported a scavenge pump for the 2 turbos, depending on their positioning (below oil level) but most likely 2 large (-12 an) drain hoses w/ inlets on the oil pan. So, does your pan still have these 2 hookups? The heads..what about them is set up for turbos? The only things that would be ideal for turbos are sodium filled exh. valves, probably polished or even ceramic coated chambers and exhaust ports..that's it. The only other thing of interest is the compression ratio but small heads & dished pistons make more power than large chambers and flat tops (less exposure to coolant and a lot more swirl in the smaller chamber)
The complexity isn't too bad, selecting the proper turbine & compressor trims is much more difficult than plumbing the system & hooking up the oil delivery & drain systems.
The underhood heat is also not that much of a problem, the only thing to get scorching hot is the turbine and you can insulate that, the turbine draws it's energy (spooling up) from the exhaust gas pressure. heat and velocity, the turbine expands the gas, it's an adiabatic expansion where little to no energy is lost to the surrounding (this is theoretical) so the exhaust gas actually makes a drop in temp in the turbine, the downpipe won't be as hot as the turbine but it's a single big pipe and easy to insulate. There are insulation ktis available for the turbine and a heat shield is easily made.
The turbine speed control is quite simple also, the easiest way is a simple spring loaded wate gate, pressure settings depend on installed spring. Then there's athe adjsutable spring loaded type where you preload the spring w/ a set screw and there's also the multiple port adjsutable spring loaded type where you can set base pressure w/ the screw and adjsut it by differential pressure over 2 ports (connected to 2 sides of a diaphragm), this allows you to hook 1 side to manifold pressure and the other to a regulator and turning the regulator will allow you to adjust the differential pressure to adjust boost pressure (where the spring setting is either maximum or minimum pressure, depending on how you hook the 2 ports up, if you swap which one is connected to manifold & regulator it works the other way around so either max or min. pressure)
Are you sure the seller didn't mean the car is equipped w/ so called turbo heads? They are a version of the double hump small chamber fuelie heads, the 3917292, or 340292. The 1st is a GM production head and has a straight plug, small valves (1.94/1.5) & 64 cc chambers, the 2nd is an over the counter head, angle plugs and large valves (2.02/1.6) ..these heads are known as turbo heads.
As for turbo packages, nothing that's on the market today will fit the C3. The banks kit is a nice kit but the log manifodls will not clear, the wastegate points forward and it will interfere with the upper control arms, especially on the pass. side because of engine shift in frame (the engine sits more towards the pass. side)
