Regarding Ohlins rear shock.
Ohlins USA’s web site does not show “DU321” anymore. I suggest contacting Ohlins USA directly and ask about availability of this particular shock and / or your options.
Ohlins USA’s site shows sales rep’s name and phone #.
http://www.ohlinsusa.com/motorcycle-groupI have Ohlins shock on my bike. And I strongly recommend buying it from your local Ohlins dealer, because you want the shock with correct spring rate (and valving) for your weight and riding style.
If things don’t work out, you can look at Penske shocks.
http://shop.penskeshocks.com/Motorcycle/http://shop.penskeshocks.com/files/downloads/PENSKE%20MC%20APPLICATION%20CHART.pdfAs for S4R’s front forks.
I have Ohlins valve kit with stock springs. Stock springs work ONLY BECAUSE I weigh only 130 lb. with full gear. (BTW, because of this “weight problem”, I can never make stock rear shock spring work for me.)
For the rider under 145lb with full gear, stock springs work just fine. Just get valve kit.