Some interesting points..
I am, sort of, in the same boat as OP.
I am planning to go na-t. my plan is to keep my w58 to start with but I will do fuel, ffim, spark upgrades, stand alone ECU from the get go. When the gearbox breaks I will look at my options. Was thinking m3 or 350z to be 6 speed but not sure how it will match the diff. Not done a lot of research on that yet. And then hopefully will be just a matter of re-mapping/upping the boost. That's my thinking anyways.
My main reasoning for this is that I know my engine is healthy and since the car really is a weekend toy I would like to have this sense of occasion/theater when driving it.
The spec I have in mind at the moment is stock engine, gte gasket, apr bolts, tubular manifold (not sure which one yet), front facing intake manifold I have already (off a member on here but not sure on brand), q45 throttle body, front mount intercooler, fuel pump, injectors, I have a link G2+ but not sure if I will use that / depends on mapper, turbosmart 45mm wastegate, BWS360 turbo (not 100% on that yet). All fitted by a reputable garage / not me lol.
What I want to achieve really is a fast spooling supra that makes all the right noises. To be honest I never had anything this fast so this is another reason why I want to keep the w58. A bit of a learning curve. Well.. cost also.