User blog comment:SlatBH/Route to Endurance Legends/@comment-36901230-20190927092445/@comment-36901230-20190930153048

Now I recall why I have the Veyron att PR 81. I wanted to go through Zenith Series with only that car. After upgrading to 81, getting all tiers unlocked, I realised that I couldn't run the last cups even with a FU Veyron, due to a too high required PR. I was then lacking one trophy to unlock Vertex Spirit, and so had to buy the Zonda R to run its showcase race.

So the partial upgrades strategy is correct, it means that to unlock Vertex Spirit, one needs to get the Veyron, the Zonda R with showcase discount, and spend 75GC to get it to PR 81 (or slightly above). That's 395GC.

By buying the Agera R and fully upgrading it with R$, no need for any other car since it can get enough trophies to unlock the next serie, But it's more expensive.