Assuming this is for a BHC or ATL? Anecdotally the Bosch diesel pump option is generally considered better than the LUK. However I'm almost certain this only really works with the AMF rather than the two newer engines due to some plumbing differences around the EGR area in the latter that have to be worked around if the Bosch pump is used. If you can do a seal replacement / refurb on the existing one then this would be an academic problem.
Actually replacing the pump itself isn't too bad - remove engine cover & boost pipe, undo 4 bolts (2 spline & 2 allen) and 2-3 pipes, remove, clean up spilled diesel, put on new gasket, connect pipes to new pump and bolt it on - with the correct torques and hope that this seals it in place. If it has been leaking then you need to look at the condition of the coolant pipes underneath there as these go sticky/soft when given a good soaking.
You can probably tell from the lack of accurate detail above that I'm not a mechanical expert - but it is do-able with the right tools and some care. Bosch pump was over £100 if I remember but at least I knew it was a good one going on.