Rewards: 74 Gold, R$74,000
Location: SEASON 1
Unlocks from: Own Any Car In This Series
Maximum Required PR: 90.9

There are 45 events - 42 Trophy + 3 TT - with each car having a specific 2-event Showcase + TT.  Only the Porsche can be upgraded enough to open the last couple of cup races, but sufficiently upgraded any one car on its own can reach 75%.
Gold progress rewards are:
25%  -  9GC
50%  -  14GC  [PR 74.7 if only using Porsche, a bit less if using others]
75%  -  20GC  [PR less than 79.2]
100% -  31GC 

(As yet don’t think we know what PR increase each Porsche upgrade represents, but if fully-upped (PR92.2) costs 1,158GC it will probably be 800-900GC for the series PR90.9)