if you want to chat on AIM sometime, I might be able to help out more, but here's afew of my thoughts on the questions...
SR ignitor fits but you need to re-wire your plugs, so why? CA ignitors are like half the price of SR ones, or you can get a pulsar or Q45 one from a junkyard for free (5 finger discount

)
If you're "building" the motor, don't cheap out with OEM 300z pistons! I don't know if they work, but I wouldn't say they are too much of an upgrade. There are afew companies starting to carry CA pistons, so grab a set from one of them:)
Wiring is easy, so you could probably do it yourself, but if you insist on sending it somewhere, I know a local guy that does amazing harness work for good prices... he does all Enjuku's harnesses/swap work.
Also, I don't know why you don't think you'll have to custom make things anymore, because there is almost NO "off the shelf" available CA performance parts... its nowhere near SR parts availability yet, and you're more than likely going to wait months for parts from Japan... so just continue making those custom stuff like you did with your KAT;)
what kind of manifold do you want? Dee (boost boy) has a shop in Miami making custom tubular t3 manifolds (Search for pictures and pricing) for around $400 IIRC. the t3/t4 setup is what most people recommend for CA's. There was a big arguement on the GT25R vs. T3/T4 afew months ago that is worth reading (again search for it in here) I don't have much experience with different turbo's as I am running an SR turbo, which was a bolt on affair, so I won't comment on which one's I think are better.
JGS tools manifolds are pretty well liked, but they don't stock our CA flange, so it'll be a little more pricey than the KA manifolds... SSAutoChrome just started making CA manifolds, but they're bottom mount t2 flanges, and are not equal length. I also am somewhat iffy about the quality of their stuff.
About the only shop that carries CA stuff is SPL parts, and you'll have to talk to Alan about them. They are a Tomei dist. but they might be able to get JUN, HKS, etc... any parts will be coming from japan, so again, expect a lengthy wait!
Welcome to the CA section:) make sure you read the old threads... there is SO much info in them!
Sean