Grennan is spot on.
I bought a fibreglass bodykit for my VS last year, spent weeks just trying to get it to fit, drilling holes, making brackets, filling imperfecions in the mould. once painted and clear-coated i found that the 'flat' areas were mostly slightly warped and reflected in every direction - completly contrasting with the rest of the car. (even if you finally manage to fit the thing properly)
Once fitted, cracks can appear in the weaker sections, and a slight knock will cause more cracks to develop and spread until you take them off to repair and repaint them. - then, oops, one of your drill holes won't line up anymore..
In short, fiberglass kits will keep you busy and constantly repairing them. I would not dream of putting a fiberglass kit on my car now and I laugh when i see them on other cars.
I would save yourself the labour and costs and stick with genuine parts. - even if you need to spend more, you will save in the long run.