The usual extra info ...
There are 51 events (48 Trophy + 3 TT) and the rewards 25-50-75-100% are split 8-13-18-27GC.  You will need all three cars for 100% and only the Porsche can be upgraded enough to complete the series.  The Vantage on its own won't reach 75% even if fully upgraded.  The Huracán will, and in fact doesn't have to be fully upped...but since mine is, I can only guess that it would need to be at PR61.5 or a bit more.  With both cars I got this far:

 

The  final three cup races are PR68.1/69.2/70.7, and I believe it will cost 205GC more to upgrade the Porsche enough (over and above the initial LTS-winning cost which apparently will be 204 cost -75 reward = 129GC nett).