Hi Nick,
Hope all good with you.
I have not had experience with these products but I did have my kit fitted 18 years ago and my recollections of that time might be helpful.
I have attached the job sheet from when I had the kit installed and you can tell from the labour charge that it was 18 years ago! The eBay kit only appears to mention the front spoiler and the rear diffuser but there are a good number of other parts required to complete the installation for example, the fixing kit, the number plate backing section and there are also two pieces of trim that go either side of the number plate. Chandlers actually overlooked these themselves but supplied, painted and fitted them anyway whilst still honouring the quoted price, so I was lucky as they would have added another hundred pounds or so.
I think what I am trying to say is that whilst the headline price of £185 per part sounds quite reasonable, by the time you add all the other bits and pieces you might find that the genuine kit (if available) is not that much more especially if you can negotiate a car club discount or similar. The painting will be what it is and will likely be a good few hundred quid. A man of your calibre will I'm sure, have no problem fitting the bits. You will also realise that to complete the M Tech look the mirrors should be changed but they are seriously expensive and I couldn't justify them at the time; having said that it seems to be widely accepted that the dumbos give far better visibility.
Hope this is of some use in your deliberations but in any case it was nice to see your name pop up
All the best
Graham