I think TBH it would be best to go for a complete system, including manifold. You could probably get one that gives a few extra horses, a nicer sound, all parts new, and all for less than vauxhall charge for something standard.
Maybe someone can suggest a few places you could get quotes from.
Mild steel is a lot cheaper than stainless, hence the price, but wont last as long due to rust.
Stainless also looks better but that is purely cosmetic because you can get perfomance exhausts in either grade metal.
If the car is used for short journeys, then stainless will be the one to go for, but if you mainly do long journeys where the exhaust really hots up, then mild steel will still last for several years.
Example, my Sport still has the original GM mild steel exhaust and manifold, and is now coming up 7 years old and still solid.
But my journeys are never less than about 15 miles so that is probably the reason it has lasted so long.
Give Powerflow a call, they only deal with stainless systems, but the prices are good, and they are custom made. A good system shouldnt cost much more (if any) than one from Kwik fit etc.