Agreed but you start with the basics
1. Check you current engine is healthy and ready for a single, a tuner can run a compression test
2. Choose your mapper and tuner, all of them do things silghtly differently and at different budgets, some offer both services
3.I would choose one who has built many proven single Supras, there are several
4. Choose what you want to car to be like to drive/live with, street 500 bhp, with a further 100 or so to spare sounds like the right car for you
5. Get the right set up for this power, 800cc injectors maybe over kill, a T04Z also maybe too much, you want optimum spool speed and power, some of the new billet turbos are putting out great power with quick spool (you will wait to get one), the T61-T67 DBB will also give an easy 550 bhp
6. Get the right supporting mods and buy quality including manifold, downpipe, fuel pump, intercooler, wastegate, exhaust, ECU, injectors, oil cooler, catch can, AFR Gauge, EGT gauge, Oil pressure gauge
7.Cams will help with top end but as JamieP has shown the stock head will deliver good power and more than enough for your current needs
8. Build with spare cash in the bank, you will need it, think about replacing the oil pump and water pump, I think it is worth it, a CAM belt, AUX belt and crank pulley change is a good idea to and of course an engine service
I am sure I have missed out loads as I am thinking quickly, really demonstrates my point as it happens, this is a project that takes time and planning, do it right and you will have one of the fastest cars on the road, get it wrong and you will soon fall out of love with it and when you sell it will never cover the cost