Welcome to the forums, I bought my 1JZ from Montreal in Canada and had it shipped to me. All I did was google and research the company, I called each one that peaked my interest and asked questions. If your satisfied with how they answered your questions check for reviews on them. Just don't jump into buying the first thing you see. As far as everyone else I'm not to sure but a quick google search pulls up a lot of results on venders.
I ended up buying a used harness off of someone on ebay and I'll have to wire it myself to the Datsun, from what I've seen it seems to be split between spending the money and buying a new harness or using the one that comes with the engine and wiring it up yourself. In my opinion it comes down to what do you think is reasonable for a wiring harness and are you willing to pay it. There are ECU pinouts everywhere online for 2JZ & 1JZ motors it just takes time, patience and some research if you aren't familiar with wiring everyone together. Again a quick google search will bring up several different harness manufacturers some require your core harness and some don't, pricing usually reflects that. Hopefully someone else can chime in about the single turbo manifold.
A lot of the guys posting build journals have some good information check it out, from what I've seen most are willing to help you out with any questions you may have.