Rewards: 67 Gold, M$67,000
Location: ENDURANCE GT RACING
Unlocks from: Player Level 32 or Owning Any Car in the Series
Maximum Required PR: 81.2
(No car-specific events, so only one car is actually necessary to complete the series)

There are 44 events - 33 Trophy + 11 TT - shared across eleven tracks, with each track hosting three Trophy events (one a Cup) and a TT.  The track sequence runs Red Bull > Nürburgring > Daytona > Monza > Dubai > COTA > Hockenheim > Silverstone > Laguna Seca > Spa > Le Mans. 
You need to complete 2 Trophy events on a track before its TT will open, and all 3 Trophy events before the next track will open.

In the absence of any other cars, PR77.8 from the Porsche LTS will get you past 75%...

Progress Rewards split:
25% - 8GC and M$8,000      [PR below 71.2]
50% - 13GC and M$13,000      [PR below 74.0]
75% - 18GC and M$18,000      [PR76.8]
100% - 28GC and M$28,000    [PR81.2]