Good updates! I'd advise getting hold of a 2nd hand genuine R32 rear, it'd be guaranteed to fit without hassle, and if you check ebay they're often available for much less than forum prices. I got mine for 150 quid. The ebay copies probably are too good to be true in most cases!
Any reason you're going for a 3.5" downpipe rather than 3"? As far as I know, unless you're going further than k04, 3.5" might be overkill. Might be worth speaking to Niki at R-Tech about that. And I'd really recommend adding an uprated fuel pump to the list before you get it mapped. They make a big difference.
Lovely part of the country you're in by the way. I go through Aberystwyth when I visit my parents. They're just down the coast road before Fishguard.