You have to ascertain why the lack of compression?
Worst case scenario's:
It could be either cylinder head gasket, worn piston rings, burnt out valves that are leaking past, even bypass leaks via the injectors.. Either way the cylinder head really needs removing, inspecting and testing on a vacuum bench at the very least, preferably with the injectors and inlet still attached to make sure it isnt either the injectors or inlet manifold leaking also.
A mechanic can inspect the bores while the head is removed for any tell tale scoring of the bores, oil blow by and piston ring damage.
By what you describe I personally wouldn't consider fitting any new items until the head has been inspected for the lack of compression first and foremost.
Sorry but I don't want you to waste money on parts until you know for sure what you are dealing with.