The first thing I did was spend gold in the Black Market on the Viper (250 gold for 5 blueprints, it switches to $350k sometimes). This allowed me to win a lot of races as the base PR is 648.
I would recommend the gold crates over anything besides the Viper though. You do get some useless crates but you have to think about the best return for your spend.
The Special Events are a waste of time, they make them far too hard to complete in the time limit so you would just be wasting money. I stopped playing them altogether. I was just playing for currency at one point but you get more from the Tuner Trial currency event (the Viper is used for the highest one).
The best value gold purchase option is the monthly one as you get 50 gold for 30 days for £5. This is 1500 gold, which costs far more as a one-off purchase. This is enough to buy 16 gold crates. Even if just half of the crates are blueprints, they have 4-5 blueprints each so you are getting about 30-40 blueprints.
Each car needs about 20 blueprints for each stage at the beginning so it's a long grind to unlock them. I bought 5 months of gold for £25 plus a one-off £10 gold option, which had a bonus amount. This gave me somewhere over 8000 gold. I unlocked the Viper with the first 1000-1250 gold then just used it on crates (about 75 crates).
The game is designed to slow down your upgrades as much as possible so that you will be frustrated enough you will want to open your wallet for sweet relief.
One thing I neglected to begin with was Blackridge races because I thought you had to win them all but you get the rewards even when you lose races. They give out 15 gold crates there every month and the game gives out a free gold crate every 48 hours so those are good for 30 crates every month (2700 gold equivalent). You get some gold from the achievements too, some have 220 gold for a certain number of races.
A faster way to go could be to get a car upgrade pack in the store just as long as you get the right car. If you can buy a really high PR car pack (800PR or more), that alone would let you unlock the first 15 chapters of the campaign.
My recommendation would be Viper plus gold crates, with Blackridge and currency trials but ignore Special Events.
