Congratulations on your purchase mate
You've definitely gone about it the right way getting one already done, unless you have money to burn that is. I did the same when I got mine, I would have loved getting my hands dirty and building one myself but just could not justify the cost. As you said those that 'build' a car by handing over a wad of cash to someone else to do the work surely can't really claim that they built it anyway and is a very expensive way to do things. But I do appreciate that not everyone is into spannering and for those it's really the only option.
I know your gonna love owning and driving your FI'd zed, I know just how your feeling as I did when I got mine If it weren't for FI I would never have had a 350z as the n/a just didn't do it for me and I would have missed out on owning a truly great car.
Just out of interest, are you actually getting belt slip? It shouldn't really be an issue if your running the 3.33 pulley, which I thought you were. Or are you just making sure by adding the extra idler wheel?