Hi Alstan
Congratulations on taking the plunge, though with the 100 day option, is never too much of a risk.
The most "useful" upgrade I undertook to the RST pre-purchase was the black Rolhoff hub finish which in the long-term will retain its looks: the silver finish degrades signficantly within 2/3 years.
The only other significant upgrade at the time was to upgrade to Deore LX brakes which certainly do their job extremely well.
I've actually just upgraded to a Brooks B17 titanium: hard to say if its worthy of the upgrade above a standard B17 because I've not owned one before, but one thing for sure is that I don't regret it. Despite the breaking in process yet to be undertaken, I already find it as comfortable as the gel saddle, and needless to say, it enhances the look of the bike.
I've also just yesterday installed a Son Dynohub in conjunction with a Supernova E3 front and rear light. Again, it's early days but from the test ride I undertook last night, it's looking to be money well spent: no noticeable resistance from the hub and the lights look well-equipped to cope with total darkness, though have yet to do the acid test on this one.
Rims: I actually stuck with the CR18 for the dynohub, from other feedback on the forum these seem to have a life of around 6,000 to 8,000 miles which did initially strike me as a little short, but having bought an RST instead of the tour, I see little point in going for a more hard wearing rim and compromising on the slender weight advantage of the RST. The CR18s have performed fine and proved to be extremely reliable.
Avoid Shimano replacement brake blocks at all costs: the first replacement set I bought started enthusiastically shredding the rims. Aztecs seem to be much kinder and as effective.
Tyres: I had panracers which performed fine at first, but I later discovered, have very thin side-walls and my rear tyre started to split on only its 2nd tour. I've replaced them with Schwalbe Marathon Racers, which though expensive, have been impressive and have thicker side-walls! I've used schwalbes on nearly on my tours and they've never failed me.
Hope this helps!
Fred
PS, If it's of any reassurance, I also bought blind and never once regretted my decision.