Rewards: 76 Gold, R$76,000
Location: SEASON 1
Unlocks from: Own Any Car In This Series
Maximum Required PR: 88.4
There are 51 events - 48 Trophy + 3 TT - with each car having a specific 2-event Showcase + TT. Only the Bentley can be upgraded enough to open the last couple of cup races, but sufficiently upgraded any one car on its own can reach 75% - in this case FU'd McLaren...
(McLaren along with FU'd Jaguar gets to 90%)
Gold progress rewards are:
25% - 9GC
50% - 15GC [PR ~78.2 if only using McLaren or Jaguar, a bit more if using only Bentley]
75% - 20GC [PR ~83.5]
100% - 32GC
Once available, all 3s for the Bentley will cost 495GC and should presumably be enough to finish its LTS (which gives back 85GC). On top of that I'm guessing you'll need to spend roughly 500GC more to finish this series.