Hi, I try to get through this step by step.
1) First get a Skyline M Spec Nur like what the TT leaderboard is using. It is available from Used Car Dealer at times. It is worth about 0.5-1sec faster than when I was using Skyline V spec Nur from Premium Car dealer without major tuning. If not, just buy the Skyline V Spec from Nissan Car dealer.
2) Buy everything that does not increase PP like flywheel, customisable transmission, customisable suspension..
3) Buy parts that increase PP and Horsepower. Buy cheaper parts like racing filter, cat converter, ECU tuning if you tight on credits. Get it to close to 480pp but much more. If it exceed a little, use engine limiter to lower the PP to 480.
3) Change transmission to lower top speed to 230kph. You are unlikely to exceed that.
4) Change brake balance to 5:4 to reduce chance of back sliding. If its still too difficult to handle change Traction Control to 1 otherwise 0 is better. Make sure ASM is off. You can turn Assisted steering to strong to help control oversteer as well.
5) Use a driving line as a guide. Downloading top ghost is unhelpful for you because you'll not see it by first 2-3 corners if I gauge your current level correctly. You can watch their replay if it helps to see where they deviate from driving line. Try not to brake at every red line... just lower the gear and test if you can get trhough with either light brake or engine braking.
6) Keep trying. Gold time is not difficult for this TT. Getting 1st in TT? Not for me in this lifetime unless no one else plays GT5 anymore.
With this setup minus the TC, Assisted steering, I am currently at 1:29.xxx. No complicated tuning as yet.